[release-branch.go1.9] cmd/compile: fix sign-extension merging rules

If we have

  y = <int16> (MOVBQSX x)
  z = <int32> (MOVWQSX y)

We used to use this rewrite rule:

(MOVWQSX x:(MOVBQSX _)) -> x

But that resulted in replacing z with a value whose type
is only int16.  Then if z is spilled and restored, it gets
zero extended instead of sign extended.

Instead use the rule

(MOVWQSX (MOVBQSX x)) -> (MOVBQSX x)

The result is has the correct type, so it can be spilled
and restored correctly.  It might mean that a few more extension
ops might not be eliminated, but that's the price for correctness.

Fixes #21963

Change-Id: I6ec82c3d2dbe43cc1fee6fb2bd6b3a72fca3af00
Reviewed-on: https://go-review.googlesource.com/65290


Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-on: https://go-review.googlesource.com/70986


Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>