[InstCombine] Add transforms for `(icmp (xor X, Y), X)`

[goldstein.w.n]

Summary

There are a variety of cases we can simplify either saving
instructions are creating sequences that are easier to optimize
elsewhere.

(X ^ Y) u> X --> X & MSB(Y) == 0

• https://alive2.llvm.org/ce/z/R63Nq2

(X ^ Y) u< X --> X & MSB(Y) != 0

• https://alive2.llvm.org/ce/z/5s9SWj

(X ^ Pos_Y) s> X --> X & MSB(Pow2_Y) == 0

• https://alive2.llvm.org/ce/z/P6VTSm

(X ^ Pos_Y) s< X --> X & MSB(Pow2_Y) != 0

• https://alive2.llvm.org/ce/z/etN_pt

(X ^ Neg_Y) s> X --> X s< 0

• https://alive2.llvm.org/ce/z/DmLgQg

(X ^ Neg_Y) s< X --> X s>= 0

• https://alive2.llvm.org/ce/z/36iZDw

[goldstein.w.n]

Rebase

[goldstein.w.n]

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[goldstein.w.n]

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[goldstein.w.n]

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[nikic]

Is there any real-world motivation for these xor transforms (and also the signed predicate variants in this patch stack)? If not, I'm not sure this is worthwhile.