[DAG] Fold select_cc setgt X, -1, C, ~C -> xor (ashr X, BW-1), C

Given a select_cc producing a constant and a invertion of the constant
for a comparison more than zero, we can produce an xor with ashr
instead, which produces smaller code. The ashr either sets all bits or
clear all bits depending on if the value is negative. This is then xor'd
with the constant to optionally negate the value.
https://alive2.llvm.org/ce/z/DTFaBZ

This includes a OneUseCheck on the Cmp, which seems to make thinks a
little worse and will be removed in a followup.

Differential Revision: https://reviews.llvm.org/D109149