[llvm] [InstCombine] Fold (A & B) - (A & ~B) --> B - (A ^ B) (PR #79717)

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AtariDreams wrote:

> Did you encounter this pattern in real-world code?

Yes: X & -(alignment - 1) is a common way to align. X & (alignment - 1) is a common way to get the lower bits starting from alignment and lower.

By subtracting these two, you get how much is between X and X rounded to alignment.

https://github.com/llvm/llvm-project/pull/79717