Commit 10f75748 (CL 32222) taught AMD64 backend to rewrite series of
byte loads or stores with corresponding shifts into a single long or
quad load or store + appropriate BSWAP. However it did not added test
for stores - only loads were tested.

Fix it.

NOTE Tests for indexed stores are not added because 10f75748 did not add
support for indexed stores - only indexed loads were handled then.

Change-Id: I48c867ebe7622ac8e691d43741feed1d40cca0d7
Reviewed-on: https://go-review.googlesource.com/34634Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>