FIR: Rework processing AugmentedArraySetCall

Previously (few commits earlier), it contained two versions
of receiver (lhs) generated separately for each desugaring version
that looked a bit redundant.

Now, at FIR building stage we just don't create desugaring sub-trees,
instead they are being built during bodies transformation and that seems
to be much convenient there, since we don't need to reverse-engineer
get-set-operator version to check if containing calls are successful
(as we just built those calls and retain them)

Semantically, this changes may only change how data flow works
for such statements (see changed compatibilityResolveWithVarargAndOperatorCall.kt)

^KT-50861 Relates

compiler/testData/diagnostics/tests/controlFlowAnalysis/deadCode/deadCodeInArrayAccess.fir.kt

@@ -34,7 +34,7 @@ fun testArrayAssignment4(n: Nothing) {
 
 fun testArrayPlusAssign(array: Array<Any>) {
     operator fun Any.plusAssign(a: Any) {}
-    array<!UNREACHABLE_CODE!>[<!>1<!UNREACHABLE_CODE!>] +=<!> todo()
+    array[1] <!UNREACHABLE_CODE!>+=<!> todo()
 }
 
 fun todo(): Nothing = throw Exception()