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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://golang.org/cl/180110044