cmd/internal/obj/arm: fix constant decomposition

cmd/internal/obj/arm:  fix constant decomposition

There are two issues in constant decomposition.

1. A typo in "func immrot2s" blocks "case 107" of []optab be triggered.

2. Though "ADD $0xffff, R0, R0" is decomposed to "ADD $0xff00, R0, R0" and
   "ADD $0x00ff, R0, R0" as expected, "ADD $0xffff, R0" still uses the
   constant pool, which should be the same as "ADD $0xffff, R0, R0".

This patch fixes them and adds more instruction encoding tests.

fix #20516

Change-Id: Icd7bdfa1946b29db15580dcb429111266f1384c6
Reviewed-on: https://go-review.googlesource.com/44335
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>