It is not clear to me what the right implementation is.
LoadReg8 and StoreReg8 are introduced during regalloc,
so after the amd64 rewrites. But implementing them
in genValue seems silly.

Change-Id: Ia708209c4604867bddcc0e5d75ecd17cf32f52c3
Reviewed-on: https://go-review.googlesource.com/12437Reviewed-by: Keith Randall <khr@golang.org>