FIR: Allow selector of safe calls to be a FirStatement

It's necessary to allow a?.b += v be interpreted as a?.(b += v)
But currently FirAssignmentOperatorStatement is not FirQualifiedAccess

^KT-41034 In Progress

compiler/fir/raw-fir/raw-fir.common/src/org/jetbrains/kotlin/fir/builder/BaseFirBuilder.kt

@@ -966,7 +966,7 @@ abstract class BaseFirBuilder<T>(val baseSession: FirSession, val context: Conte
         value: FirExpression,
         annotations: List<FirAnnotation>
     ): FirSafeCallExpression {
-        val nestedAccess = safeCallNonAssignment.regularQualifiedAccess
+        val nestedAccess = safeCallNonAssignment.selector as FirQualifiedAccess
 
         val assignment = buildVariableAssignment {
             source = baseSource
@@ -976,7 +976,7 @@ abstract class BaseFirBuilder<T>(val baseSession: FirSession, val context: Conte
             this.annotations += annotations
         }
 
-        safeCallNonAssignment.replaceRegularQualifiedAccess(
+        safeCallNonAssignment.replaceSelector(
             assignment
         )
 

compiler/fir/raw-fir/raw-fir.common/src/org/jetbrains/kotlin/fir/builder/ConversionUtils.kt

@@ -43,7 +43,6 @@ import org.jetbrains.kotlin.fir.types.impl.*
 import org.jetbrains.kotlin.lexer.KtTokens
 import org.jetbrains.kotlin.name.Name
 import org.jetbrains.kotlin.name.SpecialNames
-import org.jetbrains.kotlin.psi.KtAnnotated
 import org.jetbrains.kotlin.types.ConstantValueKind
 import org.jetbrains.kotlin.types.expressions.OperatorConventions
 import org.jetbrains.kotlin.util.OperatorNameConventions
@@ -583,7 +582,7 @@ fun FirQualifiedAccess.wrapWithSafeCall(receiver: FirExpression, source: KtSourc
         this.checkedSubjectRef = FirExpressionRef<FirCheckedSafeCallSubject>().apply {
             bind(checkedSafeCallSubject)
         }
-        this.regularQualifiedAccess = this@wrapWithSafeCall
+        this.selector = this@wrapWithSafeCall
         this.source = source
     }
 }