[dev.power64] 6g,8g: remove unnecessary and incorrect reg use scanning

Previously, the 6g and 8g registerizers scanned for used
registers beyond the end of a region being considered for
registerization.  This ancient artifact was copied from the C
compilers, where it was probably necessary to track implicitly
used registers.  In the Go compilers it's harmless (because it
can only over-restrict the set of available registers), but no
longer necessary because the Go compilers correctly track
register use/set information.  The consequences of this extra
scan were (at least) that 1) we would not consider allocating
the AX register if there was a deferproc call in the future
because deferproc uses AX as a return register, so we see the
use of AX, but don't track that AX is set by the CALL, and 2)
we could not consider allocating the DX register if there was
a MUL in the future because MUL implicitly sets DX and (thanks
to an abuse of copyu in this code) we would also consider DX
used.

This commit fixes these problems by nuking this code.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/174110043