cmd/9g: use REGZERO instead of REG_R0 if we want the zero register

cmd/9g: use REGZERO instead of REG_R0 if we want the zero register

This will make the intention clearer.
This is migrated from pre-c2go CL 4930.

Change-Id: I9103126a05323daedd729a43b94b2be8cd7408c9
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/7410
Reviewed-by: Austin Clements <austin@google.com>