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[InstCombine] enhance freelyNegateValue() by handling xor

Authored by spatel on Apr 1 2020, 11:32 AM.

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[InstCombine] enhance freelyNegateValue() by handling xor

Negation is equivalent to bitwise-not + 1, so try to convert more
subtracts into adds using this relationship:
0 - (A ^ C) => ((A ^ C) ^ -1) + 1 => A ^ ~C + 1

I doubt this will recover the regression noted in rGf2fbdf76d8d0,
but seems like we're going to need to improve here and/or revive D68408?

Alive2 proofs:
http://volta.cs.utah.edu:8080/z/Re5tMU
http://volta.cs.utah.edu:8080/z/An-uns

Differential Revision: https://reviews.llvm.org/D77230

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