[llvm] [InstCombine] Fold xor-lshr sign-mask idiom to ashr (PR #213954)

[via llvm-commits]

GabrielKimm wrote:

@dtcxzyw, could you please take a look when you have time? This adds an InstCombine fold for `((X ^ SignMask) lshr ShiftAmt) ^ SignMask` to `ashr`, and your experience with related shift and bitwise folds would be very helpful. Thank you!

https://github.com/llvm/llvm-project/pull/213954