[FIR] Don't miss smartcast info on equalities with when subjects

Since they are not `FirSmartCastExpression`s,
they are treated as proper original types,
thus leading to reporting errors instead of
warnings.

^KT-60095 Fixed

compiler/testData/diagnostics/testsWithStdLib/equalityCompatibilityCommonCases.fir.kt

@@ -53,7 +53,7 @@ fun incompatibleTypes(a: Int) = when(a) {
 fun incompatibleTypesSmartCast(a: Any?) {
     if (a is Int) {
         when(a) {
-            <!INCOMPATIBLE_TYPES!>C(10)<!> -> 1
+            <!INCOMPATIBLE_TYPES_WARNING!>C(10)<!> -> 1
             else -> 2
         }
     }