[SCEV] Prove implication of predicates to their sign-flipped counterparts

This patch teaches SCEV two implication rules:

  x <u y && y >=s 0 --> x <s y,
  x <s y && y <s 0 --> x <u y.

And all equivalents with signs/parts swapped.

Differential Revision: https://reviews.llvm.org/D110517
Reviewed By: nikic