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[InstCombine] fold extract+insert into identity shuffle

Authored by spatel on Sun, Sep 8, 12:03 PM.

Description

[InstCombine] fold extract+insert into identity shuffle

This is similar to the existing fold for splats added with:
rL365379

If we can adjust the shuffle mask to include another element
in an identity mask (if it changes vector length, that's an
extract/insert subvector operation in the backend), then that
can eliminate extractelement/insertelement pairs in IR.

All targets are expected to lower shuffles with identity masks
efficiently.

llvm-svn: 371340

Details