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>