arm/armasm: fix wrong argument order of ARM's MLA-like instructions

As reported in issue #20096, "MLS R1, R2, R3, R4" is disassembled
to "MLS R3, R1, R2, R4". This patch fixes this type of error
in all MLA-like instructions. And also test cases are added
in the decode test.

Updates #20096

Change-Id: I7f2140e9d98e8f7694892f4ea8cce87f1d2854e6
Reviewed-on: https://go-review.googlesource.com/42171
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
2 files changed