The current code calculates register number incorrectly.

The fix corrects the register number calculation.

Add cases created by decoder to test assembler.

Fixes #20697
Fixes #20723

Change-Id: I73ac153df9ea9f51c43a5104828d7a5389551c92
Reviewed-on: https://go-review.googlesource.com/45850
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>