[x86] Make the split-and-lower routine fully generic by relaxing the
assertion, making the name generic, and improving the documentation.

Step 1 in adding very primitive support for AVX-512. No functionality
changed yet.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218584 91177308-0d34-0410-b5e6-96231b3b80d8
[x86] Add the dispatch skeleton to the new vector shuffle lowering for
AVX-512.

There is no interesting logic yet. Everything ends up eventually
delegating to the generic code to split the vector and shuffle the
halves. Interestingly, that logic does a significantly better job of
lowering all of these types than the generic vector expansion code does.
Mostly, it lets most of the cases fall back to nice AVX2 code rather
than all the way back to SSE code paths.

Step 2 of basic AVX-512 support in the new vector shuffle lowering. Next
up will be to incrementally add direct support for the basic instruction
set to each type (adding tests first).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218585 91177308-0d34-0410-b5e6-96231b3b80d8
[x86] Improve naming and comments for VSELECT lowering.

No functionality changed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218586 91177308-0d34-0410-b5e6-96231b3b80d8