go/types, types2: add missing Unalias calls in type unifier

The unification code has "early exits" when the compared
types are pointer-identical.

Because of Alias nodes, we cannot simply compare x == y but we
must compare Unalias(x) == Unalias(y). Still, in the common case
there are no aliases, so as a minor optimization we write:

        x == y || Unalias(x) == Unalias(y)

to test whether x and y are (pointer-) identical.
Add the missing Unalias calls in the place where we forgot them.

Fixes #67872.

Change-Id: Ia26ffe7205b0417fc698287a4aeb1c900d30cc0d
Reviewed-on: https://go-review.googlesource.com/c/go/+/591975
Reviewed-by: Robert Findley <[email protected]>
Auto-Submit: Robert Griesemer <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>

src/cmd/compile/internal/types2/unify.go

@@ -344,7 +344,7 @@ func (u *unifier) nify(x, y Type, mode unifyMode, p *ifacePair) (result bool) {
 		// that is a type parameter.
 		assert(!isTypeParam(y))
 		// x and y may be identical now
-		if x == y {
+		if x == y || Unalias(x) == Unalias(y) {
 			return true
 		}
 	}

src/internal/types/testdata/fixedbugs/issue67872.go

@@ -0,0 +1,14 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type A = uint8
+type E uint8
+
+func f[P ~A](P) {}
+
+func g(e E) {
+	f(e)
+}