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

[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>