[X86] Add a VBROADCAST_LOAD ISD opcode representing a scalar load broadcasted to a vector.

Summary:
This adds the ISD opcode and a DAG combine to create it. There are
probably some places where we can directly create it, but I'll
leave that for future work.

This updates all of the isel patterns to look for this new node.
I had to add a few additional isel patterns for aligned extloads
which we should probably fix with a DAG combine or something. This
does mean that the broadcast load folding for avx512 can no
longer match a broadcasted aligned extload.

There's still some work to do here for combining a broadcast of
a broadcast_load. We also need to improve extractelement or
demanded vector elements of a broadcast_load. I'll try to get
those done before I submit this patch.

Reviewers: RKSimon, spatel

Reviewed By: RKSimon

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68198

llvm-svn: 373349