[FIR] Fix augmented array set call on smartcasted receiver

#KT-60917 Fixed

compiler/testData/diagnostics/tests/AssignToArrayElement.kt

@@ -1,6 +1,9 @@
 // FIR_IDENTICAL
+// !DIAGNOSTICS: -DEBUG_INFO_SMARTCAST
+
 fun getArray(): Array<Int> = throw Exception()
 fun getList(): MutableList<Int> = throw Exception()
+fun getNullableList(): MutableList<Int>? = throw Exception()
 fun fn() {
     getArray()[1] = 2
     getList()[1] = 2
@@ -8,4 +11,9 @@ fun fn() {
     getList()[1]++
     getArray()[1] += 2
     getList()[1] += 2
+
+    val nullable = getNullableList()
+    if (nullable != null) {
+        nullable[1] += 1
+    }
 }

compiler/testData/diagnostics/tests/AssignToArrayElement.txt

@@ -3,3 +3,5 @@ package
 public fun fn(): kotlin.Unit
 public fun getArray(): kotlin.Array<kotlin.Int>
 public fun getList(): kotlin.collections.MutableList<kotlin.Int>
+public fun getNullableList(): kotlin.collections.MutableList<kotlin.Int>?
+

compiler/testData/diagnostics/testsWithStdLib/smartcasts/unsoundSmartcast.fir.kt

@@ -4,7 +4,7 @@
 fun test1() {
     var x: MutableList<Int>? = mutableListOf(1)
     x!!
-    x[if (true) { x = null; 0 } else 0] <!UNRESOLVED_REFERENCE!>+=<!> <!UNSAFE_CALL!>x[0]<!>
+    x[if (true) { x = null; 0 } else 0] += <!UNSAFE_CALL!>x[0]<!>
     <!UNSAFE_CALL!>x[0]<!>.inv()
 }