[dev.ssa] cmd/compile: identical values are the same pointer

Forgot the obvious case.  Allows us to remove the load in:

func f(p *int, x int) int {
	*p = x + 5
	return *p
}

Change-Id: I93686d8240bab3a1d166b88e224cf71e3d947aef
Reviewed-on: https://go-review.googlesource.com/19905
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -204,6 +204,9 @@ func uaddOvf(a, b int64) bool {
 
 // isSamePtr reports whether p1 and p2 point to the same address.
 func isSamePtr(p1, p2 *Value) bool {
+	if p1 == p2 {
+		return true
+	}
 	// Aux isn't used  in OffPtr, and AuxInt isn't currently used in
 	// Addr, but this still works as the values will be null/0
 	return (p1.Op == OpOffPtr || p1.Op == OpAddr) && p1.Op == p2.Op &&