Much like D67339, adds ConstantRange handling for
when we know no-wrap behavior of the sub.
This is WIP, just barely working enough to post.
As noted in https://reviews.llvm.org/D67339#inline-629306,
there is something wrong with the exhaustive tests,
Those overflow checks in addWithNoWrap() aren't needed,
as per test coverage.
Even if i add a check `if every permutation
of ForeachNumInConstantRange()*ForeachNumInConstantRange()` says
there's overflow, then ConstantRange::*WithNoWrap() should
have returned Empty set, those overflow checks
are still not needed for add. One check is needed for sub though.
So, what is the missing check?