K2: generate Unit conversion for indexed assignment at raw FIR stage

#KT-59748 Fixed

compiler/testData/diagnostics/testsWithJsStdLib/dynamicTypes/setOperatorOnDynamic.fir.txt

@@ -7,6 +7,7 @@ FILE: setOperatorOnDynamic.fir.kt
         )
         lval x3: R|kotlin/Any| = R|kotlin/arrayOf|<R|kotlin/String|>().R|kotlin/collections/fold|<R|kotlin/String|, R|dynamic|>(R|kotlin/js/js|(String(({}))), <L> = fold@fun <anonymous>(res: R|dynamic|, key: R|kotlin/String|): R|dynamic| <inline=Inline, kind=UNKNOWN>  {
             R|<local>/res|.R|<dynamic>/set|(vararg(R|<local>/key|, String(hello)))
+            Unit
             ^ R|<local>/res|
         }
         )