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[X86] Add lowerShuffleAsBitRotate (PR44379)

Authored by RKSimon on Feb 9 2020, 1:15 PM.

Description

[X86] Add lowerShuffleAsBitRotate (PR44379)

As noted on PR44379, we didn't attempt to lower vector shuffles using bit rotations on XOP/AVX512F targets.

This patch lowers to uniform ISD:ROTL nodes - ROTR isn't supported by XOP and they are interchangeable for constant values anyway.

There might be cases where targets without ISD:ROTL support would benefit from this (expanding to SRL+SHL+OR), which I'll investigate in a future patch.

Also, non-AVX512BW targets fail to concatenate 256-bit rotations back to 512-bits (split during shuffle lowering as they don't have v32i16/v64i8 types).