[FIR] Fix K2 behavior according to RULES1

The compiler should only report diagnostics for
comparisons over builtins and identity-less types,
other incompatibilities should be reported
via inspections.

It's ok that in `equalityChecksOnIntegerTypes`
instead of `EQUALITY_NOT_APPLICABLE_WARNING` we get
`EQUALITY_NOT_APPLICABLE`, because
`ProperEqualityChecksInBuilderInferenceCalls`
is already active by default.

This change also replaces the notion of a representative superclass
with the least upper bound.
This makes complex types like
intersection/flexible transparent to
RULES1-based compatibility checks.
One way to look at it is to think
that this is an automatic way of handling
type parameters: automatic picking of
"interesting" bounds, and checking them against one another.

Note that `TypeIntersector.intersectTypes`
for `Int` and `T` where `T` is a type parameter
may return both `{Int & T}` or `null`
depending on `T`-s bounds. At the same time,
for type parameters `T` and `K` it will
always return `{T & K}`.

`ConeTypeIntersector.intersectTypes`, on the
other hand, will always return `{Int & T}`
irrespectively of the bounds. Meaning, the two
intersectors differ in corner cases.

`lowerBoundIfFlexible` call in `isLiterallyTypeParameter` is backed by
the `equalityOfFlexibleTypeParameters` test.

^KT-35134 #fixed-in-k2
^KT-22499 #fixed-in-k2
^KT-46383 #fixed-in-k2

compiler/testData/diagnostics/tests/BinaryCallsOnNullableValues.fir.kt

@@ -10,16 +10,16 @@ fun f(): Unit {
     x <!UNSAFE_OPERATOR_CALL!><<!> 1
     <!ASSIGNMENT_TYPE_MISMATCH!>x += 1<!>
 
-    x == 1
-    x != 1
+    <!EQUALITY_NOT_APPLICABLE_WARNING!>x == 1<!>
+    <!EQUALITY_NOT_APPLICABLE_WARNING!>x != 1<!>
 
     <!EQUALITY_NOT_APPLICABLE!>A() == 1<!>
 
-    x === "1"
-    x !== "1"
+    <!EQUALITY_NOT_APPLICABLE!>x === "1"<!>
+    <!EQUALITY_NOT_APPLICABLE!>x !== "1"<!>
 
-    x === 1
-    x !== 1
+    <!IMPLICIT_BOXING_IN_IDENTITY_EQUALS!>x === 1<!>
+    <!IMPLICIT_BOXING_IN_IDENTITY_EQUALS!>x !== 1<!>
 
     x..2
     x in 1..2