"ADDF F0, R1, F2" is silently accepted by the arm assembler and
assembled to the same binary code of "ADDF F0, F1, F2". So does
"CMPF F0, R1".

"ABSF F0, F1, F2" is also silently accepted and assembled to a
different instruction.

This patch reports those illegal forms and adds test cases.

fix #20464

Change-Id: I88b80dc29de24c6266ac7bf7bce1578c5adbc68c
Reviewed-on: https://go-review.googlesource.com/43931
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>