Technically a language change, this cleanup is a completely
backward compatible change that brings the boolean results
of comma-ok expressions in line with the boolean results of
comparisons: they are now all untyped booleans.

The implementation effort should be minimal (less than a
handfull lines of code, depending how well factored the
implementation of comma-ok expressions is).

Fixes #8189.

LGTM=iant, r, rsc
R=r, rsc, iant, ken
CC=golang-codereviews
https://golang.org/cl/112320045