[FIR] Add nullability to ILT. #KT-37639 Fixed

compiler/testData/codegen/box/boxingOptimization/kt5588.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 fun box() : String {
     val s = "notA"
     val id = when (s) {

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

@@ -3,13 +3,13 @@ FILE fqName:<root> fileName:/whenUnusedExpression.kt
     VALUE_PARAMETER name:b index:0 type:kotlin.Boolean
     VALUE_PARAMETER name:i index:1 type:kotlin.Int
     BLOCK_BODY
-      WHEN type=kotlin.Int origin=IF
+      WHEN type=kotlin.Int? origin=IF
         BRANCH
           if: GET_VAR 'b: kotlin.Boolean declared in <root>.test' type=kotlin.Boolean origin=null
-          then: BLOCK type=kotlin.Int origin=WHEN
+          then: BLOCK type=kotlin.Int? origin=WHEN
             VAR IR_TEMPORARY_VARIABLE name:tmp_0 type:kotlin.Int [val]
               GET_VAR 'i: kotlin.Int declared in <root>.test' type=kotlin.Int origin=null
-            WHEN type=kotlin.Int origin=WHEN
+            WHEN type=kotlin.Int? origin=WHEN
               BRANCH
                 if: CALL 'public final fun EQEQ (arg0: kotlin.Any?, arg1: kotlin.Any?): kotlin.Boolean declared in kotlin.internal.ir' type=kotlin.Boolean origin=EQEQ
                   arg0: GET_VAR 'val tmp_0: kotlin.Int [val] declared in <root>.test' type=kotlin.Int origin=null