[FIR] Forbid erroneous ===-checks

It was decided to forbid such comparisons,
as we know how `===` works. Also, added some more
test cases, just for comparison.

Reusing the proper `canHaveSubtypes()`
from `TypeUtils` prevents a breaking change
in:

- `comparingTripleWithPair.kt`
- `comparisonOfGenericInterfaceWithGenericClass.kt`

But it does lead to warnings
(instead of errors) in
`incompatibleEnumEntryClasses.kt`, which is an
unrelated mistake that will be fixed in the next
commit.

The refactoring in `canHaveSubtypes()` is purely
cosmetic - otherwise reading these conditions is hard
(and they don't fit my screen vertically).

^KT-62646
^KT-65541
^KT-57779

compiler/testData/diagnostics/testsWithStdLib/comparingDifferentSubclassesCommonInterface.kt

@@ -7,4 +7,7 @@ class B : I
 fun test(a: A, b: B) {
     <!EQUALITY_NOT_APPLICABLE!>a == b<!>
     a == b as I
+
+    <!EQUALITY_NOT_APPLICABLE!>a === b<!>
+    a === b as I
 }