[InstCombine] fold mul of zext bools to 'and'

The base case only works because we are relying on a
poison-unsafe select transform; if that is fixed, we
would regress on patterns like this.

The extra use tests show that the select transform can't
be applied consistently. So it may be a regression to have
an extra instruction on 1 test, but that result was not
created safely and does not happen reliably.