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/semantics/src/org/jetbrains/kotlin/fir/resolve/dfa/VariableStorageImpl.kt

@@ -62,7 +62,7 @@ class VariableStorageImpl(private val session: FirSession) : VariableStorage() {
     private fun FirElement.unwrapElement(): FirElement = when (this) {
         is FirWhenSubjectExpression -> whenRef.value.let { it.subjectVariable ?: it.subject }?.unwrapElement() ?: this
         is FirExpressionWithSmartcast -> originalExpression.unwrapElement()
-        is FirSafeCallExpression -> regularQualifiedAccess.unwrapElement()
+        is FirSafeCallExpression -> selector.unwrapElement()
         is FirCheckedSafeCallSubject -> originalReceiverRef.value.unwrapElement()
         is FirCheckNotNullCall -> argument.unwrapElement()
         else -> this

compiler/fir/semantics/src/org/jetbrains/kotlin/fir/resolve/dfa/util.kt

@@ -98,7 +98,7 @@ val FirElement.symbol: FirBasedSymbol<*>?
         is FirResolvable -> symbol
         is FirDeclaration -> symbol
         is FirWhenSubjectExpression -> whenRef.value.subject?.symbol
-        is FirSafeCallExpression -> regularQualifiedAccess.symbol
+        is FirSafeCallExpression -> selector.symbol
         else -> null
     }?.takeIf {
         (this as? FirExpression)?.unwrapSmartcastExpression() is FirThisReceiverExpression ||