[InstCombine] fold extract+insert into identity shuffle

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@371340 91177308-0d34-0410-b5e6-96231b3b80d8