[InstSimplify] Simplify comparison between zext(x) and sext(x)

This is picking up a loose thread from D69006: We can simplify
(zext x) ule (sext x) and (zext x) sge (sext x) to true, with
various permutations. Oddly, SCEV knows about this identity,
but nothing on the IR level does.

Differential Revision: https://reviews.llvm.org/D83081