Wrong instructions "MOVW 8(F0), R1" and "MOVW R0<<0(F1), R1"
are silently accepted, and all Fx are treated as Rx.

The patch checks all those illegal base registers.

fixes #20724

Change-Id: I05d41bb43fe774b023205163b7daf4a846e9dc88
Reviewed-on: https://go-review.googlesource.com/46132
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>