cmd/6g: allocate fewer new registers in sgen

cmd/6g: allocate fewer new registers in sgen

When possible, generate nodl/nodr directly into DI/SI
rather than going through a temporary register.

CX has already been saved; use it during trailing bytes cleanup.

Change-Id: I4ec6209bcc5d3bfdc927c5c132009bd8d791ada3
Reviewed-on: https://go-review.googlesource.com/2608
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>