FIR: tolerate comparison of an intersection of incompatible types

Previously, if user compares an `it<String, Int>` with `String`, the
checker reports it since the flattened types `[String, Int, Int]` are
incompatible. But technically, before flattening, the intersection type
actually contain the other side, so they should really be compatible.

compiler/testData/diagnostics/tests/enum/enumSubjectTypeCheck.fir.kt

@@ -32,8 +32,8 @@ fun useEn2(x: En2) = x
 fun bar(x: Any) {
     if (x is En && x is En2) {
         when (x) {
-            <!INCOMPATIBLE_TYPES!>En.A<!> -> useEn(x)
-            <!INCOMPATIBLE_TYPES!>En2.D<!> -> useEn2(x)
+            En.A -> useEn(x)
+            En2.D -> useEn2(x)
             else -> {}
         }
     }