K2: generate Unit conversion for indexed assignment at raw FIR stage

#KT-59748 Fixed

compiler/testData/codegen/box/fir/listAssignmentInWhen.kt

@@ -0,0 +1,14 @@
+// ISSUE: KT-59748
+// WITH_STDLIB
+
+fun foo(list: MutableList<Any?>, condition: Boolean): Unit = when {
+    condition -> list[0] = "OK"
+    else -> Unit
+}
+
+fun box(): String {
+    val list = mutableListOf<Any?>("FAIL")
+    foo(list, true)
+    foo(list, false)
+    return list[0] as String
+}

compiler/testData/diagnostics/tests/regressions/kt235.fir.kt

@@ -5,7 +5,7 @@ package kt235
 fun main() {
     val array = MyArray()
     val f: () -> String = <!INITIALIZER_TYPE_MISMATCH!>{
-        <!TYPE_MISMATCH!>array[2] = 23<!> //error: Type mismatch: inferred type is Int (!!!) but String was expected
+        array[2] = 23 //error: Type mismatch: inferred type is Int (!!!) but String was expected
     }<!>
     val g: () -> String = <!INITIALIZER_TYPE_MISMATCH!>{
         var x = 1
@@ -17,11 +17,11 @@ fun main() {
     }<!>
     val array1 = MyArray1()
     val i: () -> String = <!INITIALIZER_TYPE_MISMATCH!>{
-        <!TYPE_MISMATCH!>array1[2] = 23<!>
+        array1[2] = 23
     }<!>
 
     val fi: () -> String = <!INITIALIZER_TYPE_MISMATCH!>{
-        <!TYPE_MISMATCH!>array[2] = 23<!>
+        array[2] = 23
     }<!>
     val gi: () -> String = <!INITIALIZER_TYPE_MISMATCH!>{
         var x = 1

compiler/testData/diagnostics/testsWithJsStdLib/dynamicTypes/setOperatorOnDynamic.fir.txt

@@ -7,6 +7,7 @@ FILE: setOperatorOnDynamic.fir.kt
         )
         lval x3: R|kotlin/Any| = R|kotlin/arrayOf|<R|kotlin/String|>().R|kotlin/collections/fold|<R|kotlin/String|, R|dynamic|>(R|kotlin/js/js|(String(({}))), <L> = fold@fun <anonymous>(res: R|dynamic|, key: R|kotlin/String|): R|dynamic| <inline=Inline, kind=UNKNOWN>  {
             R|<local>/res|.R|<dynamic>/set|(vararg(R|<local>/key|, String(hello)))
+            Unit
             ^ R|<local>/res|
         }
         )

compiler/testData/diagnostics/testsWithStdLib/builderInference/unsafeAssignmentExtra.fir.txt

@@ -29,6 +29,7 @@ FILE: unsafeAssignmentExtra.fir.kt
     public final fun main(arg: R|kotlin/Any|, condition: R|kotlin/Boolean|): R|kotlin/Unit| {
         lval value: R|Foo<kotlin/Int>| = R|/myBuilder|<R|kotlin/Int|>(<L> = myBuilder@fun R|Foo<kotlin/Int>|.<anonymous>(): R|kotlin/Unit| <inline=NoInline>  {
             this@R|special/anonymous|.R|SubstitutionOverride</Foo.b: R|kotlin/Array<Stub (chain inference): TypeVariable(T)>|>|.R|SubstitutionOverride<kotlin/Array.set: R|kotlin/Unit|>|(Int(0), Int(123))
+            Unit
             this@R|special/anonymous|.R|SubstitutionOverride</Foo.a: R|Stub (chain inference): TypeVariable(T)|>| = Int(45)
              {
                 lval <unary>: R|kotlin/Int| = this@R|special/anonymous|.R|SubstitutionOverride</Foo.a: R|Stub (chain inference): TypeVariable(T)|>|
@@ -103,6 +104,7 @@ FILE: unsafeAssignmentExtra.fir.kt
             this@R|special/anonymous|.R|SubstitutionOverride</Foo.accept: R|kotlin/Unit|>|(String())
             this@R|special/anonymous|.R|SubstitutionOverride</Foo.a: R|Stub (chain inference): TypeVariable(T)|>| = Int(45)
             this@R|special/anonymous|.R|SubstitutionOverride</Foo.b: R|kotlin/Array<Stub (chain inference): TypeVariable(T)>|>|.R|SubstitutionOverride<kotlin/Array.set: R|kotlin/Unit|>|(Int(0), Int(123))
+            Unit
             R|<local>/baz|(this@R|special/anonymous|.R|SubstitutionOverride</Foo.a: R|Stub (chain inference): TypeVariable(T)|>|)
         }
         )

compiler/testData/diagnostics/testsWithStdLib/listAssignmentInWhen.fir.kt

@@ -1,10 +1,10 @@
 // ISSUE: KT-59748
 // FIR_DUMP
 
-fun foo(list: MutableList<Any?>, condition: Boolean): Unit = <!RETURN_TYPE_MISMATCH!>when {
+fun foo(list: MutableList<Any?>, condition: Boolean): Unit = when {
     condition -> list[0] = ""
     else -> Unit
-}<!>
+}
 
 fun bar(list: MutableList<Any?>, condition: Boolean): Unit = <!RETURN_TYPE_MISMATCH!>when {
     condition -> list.set(0, "")

compiler/testData/diagnostics/testsWithStdLib/listAssignmentInWhen.fir.txt

@@ -2,7 +2,11 @@ FILE: listAssignmentInWhen.fir.kt
     public final fun foo(list: R|kotlin/collections/MutableList<kotlin/Any?>|, condition: R|kotlin/Boolean|): R|kotlin/Unit| {
         ^foo when () {
             R|<local>/condition| ->  {
-                R|<local>/list|.R|SubstitutionOverride<kotlin/collections/MutableList.set: R|kotlin/Any?|>|(Int(0), String())
+                 {
+                    R|<local>/list|.R|SubstitutionOverride<kotlin/collections/MutableList.set: R|kotlin/Any?|>|(Int(0), String())
+                    Unit
+                }
+
             }
             else ->  {
                 Q|kotlin/Unit|