[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