FIR2IR: set 'SetField' type to Unit, convert explicit 'this' references

compiler/testData/ir/irText/expressions/safeCallWithIncrementDecrement.fir.txt

@@ -33,7 +33,7 @@ FILE fqName:test fileName:/safeCallWithIncrementDecrement.kt
     BLOCK_BODY
       RETURN type=kotlin.Nothing from='public final fun inc (): kotlin.Int? declared in test'
         CALL 'public final fun inc (): kotlin.Int declared in kotlin.Int' type=kotlin.Int origin=null
-          $this: ERROR_CALL 'Unresolved reference: this#' type=kotlin.Int?
+          $this: GET_VAR '<this>: kotlin.Int? declared in test.inc' type=kotlin.Int? origin=null
   FUN name:get visibility:public modality:FINAL <> ($receiver:kotlin.Int?, index:kotlin.Int) returnType:kotlin.Int
     $receiver: VALUE_PARAMETER name:<this> type:kotlin.Int?
     VALUE_PARAMETER name:index index:0 type:kotlin.Int