cmd/compile: fix dominator check in check()

Ancestor comparison was the wrong way around, effectively
disabling the def-must-dominate-use check.

Update #15084

Change-Id: Ic56d674c5000569d2cc855bbb000a60eae517c7c
Reviewed-on: https://go-review.googlesource.com/22330
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

src/cmd/compile/internal/ssa/check.go

@@ -338,7 +338,7 @@ func checkFunc(f *Func) {
 
 // domCheck reports whether x dominates y (including x==y).
 func domCheck(f *Func, sdom sparseTree, x, y *Block) bool {
-	if !sdom.isAncestorEq(y, f.Entry) {
+	if !sdom.isAncestorEq(f.Entry, y) {
 		// unreachable - ignore
 		return true
 	}