[PATCH][X86] Optimize shifting of result of a vector-compare

[Adam Nemet]

This change is motivated by the benchmark in SingleSource/Benchmarks/Misc/lowercase.c.

             pcmpgtw %xmm3, %xmm2
             pand    %xmm7, %xmm2      // xmm7 is <1, 1, 1, 1, 1, 1, 1, 1>
             psllw   $5, %xmm2

PCMP* sets its result to all ones or zeros so we can AND with the shifted mask rather than masking and shifting separately.

The patch adds this transformation to the DAGCombiner:

   (shl (and (setcc:i8v16 ...) N01C) N1C) -> (and (setcc:i8v16 ...) N01C<<N1C)

This is a similar transformation that is already done for sign-extending the result of setcc for targets where boolean values are either zero or all-ones.

If the patch looks good, please apply.

Adam

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