[X86] Add DAG combine to merge vzext_movl with the various fp<->int conversion operat...

[X86] Add DAG combine to merge vzext_movl with the various fp<->int conversion operations that only write the lower 64-bits of an xmm register and zero the rest.

Summary: We have isel patterns for this, but we're missing some load patterns and all broadcast patterns. A DAG combine seems like a better fit for this.

Reviewers: RKSimon, spatel

Reviewed By: RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D56971

llvm-svn: 352260