cmd/compile: benchmark needs dominator tree

Now that CSE uses dom tree to order partitions, we need the
dom tree computed before benchmarking CSE.

Fixes #15801

Change-Id: Ifa4702c7b75250f34de185e69a880b3f3cc46a12
Reviewed-on: https://go-review.googlesource.com/23361Reviewed-by: David Chase <drchase@google.com>

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

@@ -35,7 +35,7 @@ func benchFnPass(b *testing.B, fn passFunc, size int, bg blockGen) {
 	b.ReportAllocs()
 	c := NewConfig("amd64", DummyFrontend{b}, nil, true)
 	fun := Fun(c, "entry", bg(size)...)
-
+	domTree(fun.f)
 	CheckFunc(fun.f)
 	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
@@ -51,7 +51,7 @@ func benchFnBlock(b *testing.B, fn passFunc, bg blockGen) {
 	b.ReportAllocs()
 	c := NewConfig("amd64", DummyFrontend{b}, nil, true)
 	fun := Fun(c, "entry", bg(b.N)...)
-
+	domTree(fun.f)
 	CheckFunc(fun.f)
 	b.ResetTimer()
 	for i := 0; i < passCount; i++ {