Fold MOVDaddr ops into MOVXstorezero ops.
Also fold ADDconst into MOVDaddr so we're sure there isn't
(MOVDstorezero (ADDconst (MOVDaddr ..)))

Without this CL, we get:

v1 = MOVDaddr {s}
v2 = VARDEF {s}
v3 = MOVDstorezero v1 v2

The liveness pass thinks the MOVDaddr is a read of s, so s is
incorrectly thought to be live at the start of the function.

Fixes #17194

Change-Id: I2b4a2f13b12aa5b072941ee1c7b89f3793650cdc
Reviewed-on: https://go-review.googlesource.com/30086
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Michael Munday <munday@ca.ibm.com>