cmd/compile: invalidate zero-use values during rewrite

cmd/compile: invalidate zero-use values during rewrite

This helps remove uses that aren't needed any more.
That in turn helps other rules with Uses==1 conditions fire.

Update #39918

Change-Id: I68635b675472f1d59e59604e4d34b949a0016533
Reviewed-on: https://go-review.googlesource.com/c/go/+/249463
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>