K2: Avoid using Nothing? as inference result in the majority of cases

Namely, do not choose `Nothing?` result type when fixing a variable
that has other constraints besides the ones that came from
the relevant type parameter's upper bounds.

See more details in KT-55691.

In K1, the case from specialCallWithMaterializeAndExpectedType.kt
was working (inferred to String?) just because the branches
were analyzed independently with `String?` expected type.

This change became necessary after the previous commit when we united
inference subsystems for if/when branches (see motivation there).

NB: For K1, the behavior is left the same, but the code
was refactored a bit.

^KT-55691 Fixed
^KT-56448 Fixed

compiler/testData/diagnostics/testsWithStdLib/inference/annotationsForResolve/exactAnnotationWithUpperBoundConstraint.fir.kt

@@ -8,7 +8,7 @@ fun test() {
     val map: Map<String, Int> = mapOf("x" to 1)
 
     val r1 = map.getOrDefault_Exact("y", null)
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Nothing?")!>r1<!>
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int?")!>r1<!>
 
     val r2 = map.getOrDefault_Exact("y", null as Int?)
     <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int?")!>r2<!>