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

#KT-59748 Fixed

compiler/fir/analysis-tests/testData/resolve/arguments/setWithTrailingLambda.fir.txt

@@ -8,6 +8,7 @@ FILE: setWithTrailingLambda.kt
             ^ Unit
         }
         )
+        Unit
     }
     public final data class EditorData : R|kotlin/Any| {
         public constructor(meta: R|MyMap<EditorData, kotlin/Any>|): R|EditorData| {

compiler/fir/analysis-tests/testData/resolve/arrays/arraySet.fir.txt

@@ -46,10 +46,13 @@ FILE: arraySet.kt
     }
     public final fun test_1(a: R|A<B>|): R|kotlin/Unit| {
         R|<local>/a|.R|SubstitutionOverride</A.set: R|kotlin/Unit|>|(Int(0), R|/B.B|())
+        Unit
     }
     public final fun test_2(a: R|A<C>|): R|kotlin/Unit| {
         R|<local>/a|.R|SubstitutionOverride</A.set: R|kotlin/Unit|>|(Int(0), R|/C.C|())
+        Unit
     }
     public final fun test_3(a: R|A<D>|): R|kotlin/Unit| {
         R|<local>/a|.R|SubstitutionOverride</A.set: R|kotlin/Unit|>|(Int(0), R|/D.D|())
+        Unit
     }

compiler/fir/analysis-tests/testData/resolve/checkers/notUselessCast_3.fir.txt

@@ -26,6 +26,7 @@ FILE: notUselessCast_3.kt
         }
         )
         R|<local>/cards|.R|SubstitutionOverride<kotlin/collections/List.get: R|kotlin/collections/List<kotlin/collections/MutableList<Square>>|>|(Int(0)).R|SubstitutionOverride<kotlin/collections/List.get: R|kotlin/collections/MutableList<Square>|>|(Int(0)).R|SubstitutionOverride<kotlin/collections/MutableList.set: R|Square|>|(Int(0), Q|MARKED|)
+        Unit
     }
     public final fun test_2(): R|kotlin/Unit| {
         lval lines: R|kotlin/collections/List<kotlin/String>| = R|kotlin/collections/listOf|<R|kotlin/String|>()
@@ -40,4 +41,5 @@ FILE: notUselessCast_3.kt
         }
         )
         R|<local>/cards|.R|SubstitutionOverride<kotlin/collections/List.get: R|kotlin/collections/List<kotlin/collections/MutableList<UNMARKED>>|>|(Int(0)).R|SubstitutionOverride<kotlin/collections/List.get: R|kotlin/collections/MutableList<UNMARKED>|>|(Int(0)).R|SubstitutionOverride<kotlin/collections/MutableList.set: R|UNMARKED|><Inapplicable(INAPPLICABLE): kotlin/collections/MutableList.set>#|(Int(0), Q|MARKED|)
+        Unit
     }