cmd/compile: ignore dead phis in fuse

Happens occasionally for boolean phis was used as a control.

Change-Id: Ie0f2483e9004c1706751d8dfb25ee2e5106d917e
Reviewed-on: https://go-review.googlesource.com/21310
Run-TryBot: Alexandru Moșoi <alexandru@mosoi.ro>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -65,7 +65,7 @@ func fuseBlockIf(b *Block) bool {
 		b.Fatalf("invalid predecessors")
 	}
 	for _, v := range ss.Values {
-		if v.Op == OpPhi && v.Args[i0] != v.Args[i1] {
+		if v.Op == OpPhi && v.Uses > 0 && v.Args[i0] != v.Args[i1] {
 			return false
 		}
 	}