[dev.cc] 9g: fill progtable for CC, V, and VCC instruction variants

This adds some utilities for converting between the CC, V, and
VCC variants of operations and uses these to derive the
ProgInfo entries for these variants (which are identical to
the ProgInfo for the base operations).

The 9g peephole optimizer will also use these conversion
utilities.

LGTM=minux, rsc
R=rsc, dave, minux
CC=golang-codereviews
https://codereview.appspot.com/180110044
[dev.cc] 9g: peephole optimizer

This was based on the 9c peephole optimizer, modified to work
with code generated by gc and use the proginfo infrastructure
in gc.

LGTM=rsc
R=rsc, bradfitz, minux
CC=golang-codereviews
https://codereview.appspot.com/179190043