[FIR] Resolve type of when expression to Unit if it is not exhaustive

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

@@ -2,7 +2,7 @@ FILE fqName:<root> fileName:/tryCatchWithImplicitCast.kt
   FUN name:testImplicitCast visibility:public modality:FINAL <> (a:kotlin.Any) returnType:kotlin.Unit
     VALUE_PARAMETER name:a index:0 type:kotlin.Any
     BLOCK_BODY
-      WHEN type=kotlin.Any? origin=IF
+      WHEN type=kotlin.Unit origin=IF
         BRANCH
           if: TYPE_OP type=kotlin.Boolean origin=NOT_INSTANCEOF typeOperand=kotlin.String
             GET_VAR 'a: kotlin.Any declared in <root>.testImplicitCast' type=kotlin.Any origin=null