[X86] Add DAG combine to turn (vzmovl (insert_subvector undef, X, 0)) into...

[X86] Add DAG combine to turn (vzmovl (insert_subvector undef, X, 0)) into (insert_subvector allzeros, (vzmovl X), 0)

128/256 bit scalar_to_vectors are canonicalized to (insert_subvector undef, (scalar_to_vector), 0). We have isel patterns that try to match this pattern being used by a vzmovl to use a 128-bit instruction and a subreg_to_reg.

This patch detects the insert_subvector undef portion of this and pulls it through the vzmovl, creating a narrower vzmovl and an insert_subvector allzeroes. We can then match the insertsubvector into a subreg_to_reg operation by itself. Then we can fall back on existing (vzmovl (scalar_to_vector)) patterns.

Note, while the scalar_to_vector case is the motivating case I didn't restrict to just that case. I'm also wondering about shrinking any 256/512 vzmovl to an extract_subvector+vzmovl+insert_subvector(allzeros) but I fear that would have bad implications to shuffle combining.

I also think there is more canonicalization we can do with vzmovl with loads or scalar_to_vector with loads to create vzload.

Differential Revision: https://reviews.llvm.org/D63512

llvm-svn: 364095