[X86] Teach lowering to only let through (insert_subvector (vXi1 zeros),...

[X86] Teach lowering to only let through (insert_subvector (vXi1 zeros), subvec, 0) for vector sizes that have native KSHIFT support.

For narrow sizes we'll widen the zero vector and widen the insert. Then do an extract_subvector to get back down to correct size.

This allows us to remove some patterns from the isel table that had to COPY_TO_REGCLASS to an oversized register, do the shift and then COPY_TO_REGCLASS back to the narrow register. Now this is represented explicitly in the DAG.

This seems to have perturbed the register allocation in one of the tests, but the number of instructions didn't change.

llvm-svn: 320190