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[X86] Use compare with 0 to fill an element with sign bits when sign extending…

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[X86] Use compare with 0 to fill an element with sign bits when sign extending to v2i64 pre-sse4.1

Previously we used an arithmetic shift right by 31, but that requires a copy to preserve the input. So we might as well materialize a zero and compare to it since the comparison will overwrite the register that contains the zeros. This should be one byte shorter.

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Committed
ctopperNov 18 2018, 8:33 PM
Parents
rL347180: [X86] Remove most of the SEXTLOAD Custom setOperationAction calls under -x86…
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