Adds the following instructions and uses them in the SSA backend:

 - ANDW
 - ORW
 - XORW

The instruction encodings for 32-bit operations are typically shorter,
particularly when an immediate is used. For example, XORW $-1, R1
only requires one instruction, whereas XOR requires two.

Also removes some unused instructions (that were emulated):

 - ANDN
 - NAND
 - ORN
 - NOR

Change-Id: Iff2a16f52004ba498720034e354be9771b10cac4
Reviewed-on: https://go-review.googlesource.com/30291Reviewed-by: Cherry Zhang <cherryyz@google.com>