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

@@ -1595,21 +1595,21 @@ class HtmlFirDump internal constructor(private var linkResolver: FirLinkResolver
 
         +"?."
 
-        val nestedQualifier = safeCallExpression.regularQualifiedAccess
-        if (nestedQualifier.explicitReceiver == safeCallExpression.checkedSubjectRef.value) {
-            when (nestedQualifier) {
+        val selector = safeCallExpression.selector
+        if (selector is FirQualifiedAccess && selector.explicitReceiver == safeCallExpression.checkedSubjectRef.value) {
+            when (selector) {
                 is FirFunctionCall -> {
-                    return generate(nestedQualifier, skipReceiver = true)
+                    return generate(selector, skipReceiver = true)
                 }
                 is FirQualifiedAccessExpression -> {
-                    return generate(nestedQualifier, skipReceiver = true)
+                    return generate(selector, skipReceiver = true)
                 }
             }
         }
 
         +"{ "
 
-        generate(nestedQualifier)
+        generate(selector)
 
         +" }"
     }