Small optimizations I noticed while looking at Giovanni's test cases.

More shifts by constants.
Indexed stores for smaller types.
Fold LEA into loads/stores.
More extending loads.
CMP $0 of AND -> TEST

Fix order of TEST ops.

Giovanni's test cases at https://gist.github.com/rasky/62fba94e3a20d1b05b2a

Change-Id: I7077bc0b5319bf05767eeb39f401f4bb4b39f635
Reviewed-on: https://go-review.googlesource.com/19086
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Todd Neal <todd@tneal.org>
Reviewed-by: David Chase <drchase@google.com>