[FIR] Get rid of operation in FirVariableAssignment

`FirVariableAssignment` is creates only for expressions `x = y`
For other assign operators there is a `FirOperatorCall`

compiler/fir/dump/src/org/jetbrains/kotlin/fir/dump/HtmlFirDump.kt

@@ -639,9 +639,7 @@ class HtmlFirDump internal constructor(private var linkResolver: FirLinkResolver
     private fun FlowContent.generate(variableAssignment: FirVariableAssignment) {
         generateReceiver(variableAssignment)
         generate(variableAssignment.lValue)
-        +" "
-        +variableAssignment.operation.operator
-        +" "
+        +" = "
         generate(variableAssignment.rValue)
     }
 

compiler/fir/psi2fir/src/org/jetbrains/kotlin/fir/builder/BaseFirBuilder.kt

@@ -471,8 +471,8 @@ abstract class BaseFirBuilder<T>(val session: FirSession, val context: Context =
                 arguments += value
             }
         }
-
-        return FirVariableAssignmentImpl(source, false, value, operation).apply {
+        require(operation == FirOperation.ASSIGN)
+        return FirVariableAssignmentImpl(source, false, value).apply {
             lValue = initializeLValue(this@generateAssignment) { convert() as? FirQualifiedAccess }
         }
     }

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/body/resolve/FirExpressionsResolveTransformer.kt

@@ -246,7 +246,7 @@ class FirExpressionsResolveTransformer(transformer: FirBodyResolveTransformer) :
                 operatorCallReference == null || (!lhsIsVar && !assignIsError) -> resolvedAssignCall.compose()
                 assignCallReference == null -> {
                     val assignment =
-                        FirVariableAssignmentImpl(operatorCall.source, false, resolvedOperatorCall, FirOperation.ASSIGN).apply {
+                        FirVariableAssignmentImpl(operatorCall.source, false, resolvedOperatorCall).apply {
                             lValue = if (lhsIsVar)
                                 lhsReference!!
                             else

compiler/fir/tree/src/org/jetbrains/kotlin/fir/FirRenderer.kt

@@ -928,7 +928,7 @@ class FirRenderer(builder: StringBuilder) : FirVisitorVoid() {
         visitQualifiedAccess(variableAssignment)
         variableAssignment.lValue.accept(this)
         print(" ")
-        visitAssignment(variableAssignment.operation, variableAssignment.rValue)
+        visitAssignment(FirOperation.ASSIGN, variableAssignment.rValue)
     }
 
     override fun visitArraySetCall(arraySetCall: FirArraySetCall) {

compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/FirVariableAssignment.kt

@@ -25,7 +25,6 @@ abstract class FirVariableAssignment : FirPureAbstractElement(), FirQualifiedAcc
     abstract override val calleeReference: FirReference
     abstract val lValue: FirReference
     abstract val rValue: FirExpression
-    abstract val operation: FirOperation
 
     override fun <R, D> accept(visitor: FirVisitor<R, D>, data: D): R = visitor.visitVariableAssignment(this, data)
 

compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/impl/FirVariableAssignmentImpl.kt

@@ -8,7 +8,6 @@ package org.jetbrains.kotlin.fir.expressions.impl
 import org.jetbrains.kotlin.fir.FirSourceElement
 import org.jetbrains.kotlin.fir.expressions.FirAnnotationCall
 import org.jetbrains.kotlin.fir.expressions.FirExpression
-import org.jetbrains.kotlin.fir.expressions.FirOperation
 import org.jetbrains.kotlin.fir.expressions.FirVariableAssignment
 import org.jetbrains.kotlin.fir.impl.FirAbstractAnnotatedElement
 import org.jetbrains.kotlin.fir.references.FirReference
@@ -22,8 +21,7 @@ import org.jetbrains.kotlin.fir.visitors.*
 class FirVariableAssignmentImpl(
     override val source: FirSourceElement?,
     override var safe: Boolean,
-    override var rValue: FirExpression,
-    override var operation: FirOperation
+    override var rValue: FirExpression
 ) : FirVariableAssignment(), FirModifiableQualifiedAccess, FirAbstractAnnotatedElement {
     override val annotations: MutableList<FirAnnotationCall> = mutableListOf()
     override var explicitReceiver: FirExpression? = null

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/ImplementationConfigurator.kt

@@ -383,7 +383,6 @@ object ImplementationConfigurator : AbstractFirTreeImplementationConfigurator()
 
         impl(variableAssignment) {
             parents += modifiableQualifiedAccess
-            isMutable("operation")
             lateinit("calleeReference")
             defaultNoReceivers()
 

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/NodeConfigurator.kt

@@ -439,7 +439,6 @@ object NodeConfigurator : AbstractFieldConfigurator() {
         variableAssignment.configure {
             +field("lValue", reference)
             +field("rValue", expression).withTransform()
-            +field("operation", operationType)
         }
 
         whenSubjectExpression.configure {

compiler/visualizer/render-fir/src/org/jetbrains/kotlin/compiler/visualizer/FirVisualizer.kt

@@ -445,7 +445,7 @@ class FirVisualizer(private val firFile: FirFile) : BaseRenderer() {
         }
 
         override fun visitVariableAssignment(variableAssignment: FirVariableAssignment, data: StringBuilder) {
-            data.append("qualified access: \"${variableAssignment.operation}\"")
+            data.append("variable assignment")
         }
 
         override fun visitStarProjection(starProjection: FirStarProjection, data: StringBuilder) {