[PATCH] DAGCombiner: fold (xor (shl 1, x), -1) -> (rotl -2, x)

[David Majnemer]

In http://reviews.llvm.org/D8350#141699, @silvas wrote:

> Maybe a z3 proof in the commit message would make people feel a bit more at home with this also?


I sent this out for review mainly for the SelectionDAG specific bits.  I think the arithmetic behind it is pretty clear.


http://reviews.llvm.org/D8350

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