[dev.power64] cmd/9g: do not ignore move instructions even if from and to are samaddr()

Those moves might be significant (e.g. narrowing or widening): on Power64,
we don't distinguish between MOVWD and MOVW.

This fixes divmode.go and bug447.go.

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://codereview.appspot.com/125200043

Committer: Russ Cox <rsc@golang.org>