NI: remove separation of variable fixation order by constraint kind

The commit partially reverts ec4d9d2f1f

^KT-37914 Fixed

compiler/testData/diagnostics/tests/inference/commonSystem/castToSubtype.kt

@@ -0,0 +1,60 @@
+// FIR_IDENTICAL
+// !DIAGNOSTICS: -UNCHECKED_CAST
+// Issue: KT-37914
+
+interface I
+
+fun <R, U : R> castToSubtype(obj: R) = obj as U
+
+fun <T> select(vararg x: T) = x[0]
+
+fun <S> materialize(): S = null as S
+
+// Case 1 (using intermediate supertype)
+
+/*
+ Constraint system:
+     R:
+        TypeVariable(U) <: TypeVariable(R)
+        I <: TypeVariable(R)
+     U:
+        I >: TypeVariable(U)
+        Foo<TypeVariable(P)> >: TypeVariable(U)
+     T:
+        Foo<Any> <: TypeVariable(T)
+        Bar<TypeVariable(P)> <: TypeVariable(T)
+
+ Fixation order before the fix: R, U, T, P
+ Fixation order after the fix: R, T, P, U
+
+ `U` has begun to have lower priority so it can be fixed to more specific type (to `Foo<Any>` instead of `I`).
+ */
+
+interface Foo<T> : I
+
+class Bar<T>(val x: Foo<T>) : Foo<T>
+
+fun main2() {
+    select(
+        materialize<Foo<Any>>(),
+        Bar(
+            castToSubtype(materialize<I>()) // NI: "required – Foo<Any>, found – I" afther the commit, OI – OK
+        )
+    )
+}
+
+// Case 2 (using deep supertype)
+
+interface Foo1<Y> : I
+interface Foo2<Y> : Foo1<Y>
+
+class Bar1<P>(val x: Foo2<P>) : Foo2<P>
+
+fun main1() {
+    select(
+        materialize<Foo2<Any>>(),
+        Bar1(
+            castToSubtype(materialize<I>()) // NI: "required – Foo<Any>, found – I" afther the commit, OI – OK
+        )
+    )
+}

compiler/testData/diagnostics/tests/inference/commonSystem/castToSubtype.txt

@@ -0,0 +1,47 @@
+package
+
+public fun </*0*/ R, /*1*/ U : R> castToSubtype(/*0*/ obj: R): U
+public fun main1(): kotlin.Unit
+public fun main2(): kotlin.Unit
+public fun </*0*/ S> materialize(): S
+public fun </*0*/ T> select(/*0*/ vararg x: T /*kotlin.Array<out T>*/): T
+
+public final class Bar</*0*/ T> : Foo<T> {
+    public constructor Bar</*0*/ T>(/*0*/ x: Foo<T>)
+    public final val x: Foo<T>
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public final class Bar1</*0*/ P> : Foo2<P> {
+    public constructor Bar1</*0*/ P>(/*0*/ x: Foo2<P>)
+    public final val x: Foo2<P>
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface Foo</*0*/ T> : I {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface Foo1</*0*/ Y> : I {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface Foo2</*0*/ Y> : Foo1<Y> {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface I {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}