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[X86][CostModel] X86TTIImpl::getShuffleCost(): subvector insertions are cheap
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Authored by lebedev.ri on Apr 17 2021, 3:43 AM.

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Summary

This is similar to the subvector extractions,
except that the 0'th subvector isn't free to insert,
because we generally don't know whether or not
the upper elements need to be preserved:
https://godbolt.org/z/rsxP5W4sW

This is needed to avoid regressions in D100684

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