[mlir][index] Fold `cmp(max/min(x, cstA), cstB)`

This is a case that is not picked up by integer range inference and
suggests a weakness with integer range inference on the index dialect.
The problem is that when `[1, SMAX_64]` is truncated to 32 bits, the
resulting range could be `[SMIN_32, SMAX_32]`, making the subsequent
comparison worthless. This is because integer range inference doesn't
know that the result of the max/min inference also changes based on the
bitwidth, and doing the truncation locally at the input of the
comparison op loses that information.

This also was a pattern that frequently showed up in our code, so adding
it as a folder allows dead code to be pruned more frequently.

Depends on D153731

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D153732