[SCEVExpander] Skip creating <u 0 check, which is always false.

Unsigned compares of the form <u 0 are always false. Do not create such
a redundant check in generateOverflowCheck.

The patch introduces a new lambda to create the check, so we can
exit early conveniently and skip creating some instructions feeding the
check.

I am planning to sink a few additional instructions as follow-ups, but I
would prefer to do this separately, to keep the changes and diff
smaller.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D116811