[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/tests-spec/testData/diagnostics/notLinked/dfa/pos/4.fir.kt

@@ -257,7 +257,7 @@ fun case_22(a: (() -> Unit)) {
 fun case_23(a: ((Float) -> Int), b: Float) {
     if (<!SENSELESS_COMPARISON!>a == null<!> && <!SENSELESS_COMPARISON!>b == null<!>) {
         val x = <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int")!>a(<!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Float & kotlin.Nothing")!>b<!>)<!>
-        if (<!SENSELESS_COMPARISON!>x !== null<!>) {
+        if (<!FORBIDDEN_IDENTITY_EQUALS_WARNING, SENSELESS_COMPARISON!>x !== null<!>) {
             <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int")!>x<!>
         }
     }
@@ -282,7 +282,7 @@ fun case_25(a: (() -> Unit) -> Unit, b: (() -> Unit) -> Unit = if (<!SENSELESS_C
 }
 
 // TESTCASE NUMBER: 26
-fun case_26(a: Int, b: Int = if (<!SENSELESS_COMPARISON!>a === null<!>) <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int & kotlin.Nothing")!>a<!> else 0) {
+fun case_26(a: Int, b: Int = if (<!FORBIDDEN_IDENTITY_EQUALS_WARNING, SENSELESS_COMPARISON!>a === null<!>) <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int & kotlin.Nothing")!>a<!> else 0) {
     <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int")!>a<!>
     <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int")!>b<!>
 }