[SCEV] Generalize implication when signedness of FoundPred doesn't matter

The implication logic for two values that are both negative or non-negative
says that it doesn't matter whether their predicate is signed and unsigned,
but only flips unsigned into signed for further inference. This patch adds
support for flipping a signed predicate into unsigned as well.

Differential Revision: https://reviews.llvm.org/D109959
Reviewed By: nikic