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[instcombine][x86] Converted pdep/pext with shifted mask to simple arithmetic

Authored by reames on Sep 18 2020, 2:53 PM.

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[instcombine][x86] Converted pdep/pext with shifted mask to simple arithmetic

If the mask of a pdep or pext instruction is a shift masked (i.e. one contiguous block of ones) we need at most one and and one shift to represent the operation without the intrinsic. One all platforms I know of, this is faster than the pdep/pext.

The cost modelling for multiple contiguous blocks might be worth exploring in a follow up, but it's not relevant for my current use case. It would almost certainly be a win on AMDs where these are really really slow though.

Differential Revision: https://reviews.llvm.org/D87861