[InstCombine] try to fold icmp with mismatched extended operands

If a value is known to be non-negative and zexted,
that's the same thing as sexted.

So for the purpose of looking past the casts with
an icmp, treat it as if it was a sext:
https://alive2.llvm.org/ce/z/_BDsGV

This is necessary, but not enough to solve the
motivating problem:
https://github.com/llvm/llvm-project/issues/55013

Differential Revision: https://reviews.llvm.org/D124419