cmd/compile: add missing s390x load with index operations

cmd/compile: add missing s390x load with index operations

Prior to this CL loads with sign extension could not be replaced with
indexed loads (only loads with zero extension).

This CL also prevents large offsets (more than 20-bits) from being
merged into indexed loads. It is better to keep such offsets
separate.

Gives a small improvement in binary size, ~1.5KB from .text in cmd/go.

Change-Id: Ib848ffc2b05de6660c5ce2394ae1d1d144273e29
Reviewed-on: https://go-review.googlesource.com/36845
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>