"MOVW R1, CPSR" is assembled to 0xe129f001, which should be 0xe12cf001.
"MOVW $255, CPSR" is assembled to 0xe329f0ff, which should be 0xe32cf0ff.

This patch fixes them and adds more assembly encoding tests.

fix #20626

Change-Id: Iefc945879ea774edf40438ce39f52c144e1501a1
Reviewed-on: https://go-review.googlesource.com/45170Reviewed-by: Cherry Zhang <cherryyz@google.com>