Fold ashr-exact into a icmp-ugt.

Fold ashr-exact into a icmp-ugt.

This commit optimizes the code sequence:
  icmp-XXX (ashr-exact (X, C_1), C_2).

Instcombine already implements this optimization for sgt, and this
patch adds support to additional predicates. The transformation is legal
for all predicates if the 'exact' flag is set, and to SGE, UGE, SLT, ULT
when the exact flag is not present.

This pattern is found in the std::vector bounds checks code of the at()
method.

Alive2 proof:
https://alive2.llvm.org/ce/z/JT_WL8

Differential Revision: https://reviews.llvm.org/D117252