[InstCombine] Convert xor (ashr X, BW-1), C -> select(X >=s 0, C, ~C)

The sequence of instructions `xor (ashr X, BW-1), C` (or with a truncation
`xor (trunc (ashr X, BW-1)), C)` takes a value, produces all zeros or all
ones and with it optionally inverts a constant depending on whether the
original input was positive or negative. This is the same as checking if
the value is positive, and selecting between the constant and ~constant.
https://alive2.llvm.org/ce/z/NJ85qY

This is a fairly general version of a fold that helps pull saturating
arithmetic into a canonical form.

Differential Revision: https://reviews.llvm.org/D109151