cmd/compile: fix bug in phiopt pass

The math to invert the input index was wrong.

Fixes #45323

Change-Id: I7c68cac280e8f01a9c806ecb0f195f169267437e
Reviewed-on: https://go-review.googlesource.com/c/go/+/306431


Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: fannie zhang <Fannie.Zhang@arm.com>
Reviewed-by: David Chase <drchase@google.com>