[X86] Add lowering to VALIGN

This was currently part of lowering to PALIGNR with some special-casing to
make interlane shifting work.  Since AVX512F has interlane alignr (valignd/q)
and AVX512BW has vpalignr we need to support both of these *at the same time*,
e.g. for SKX.

This patch breaks out the common code and then add support to check both of
these lowering options from LowerVECTOR_SHUFFLE.

I also added some FIXMEs where I think the AVX512BW and AVX512VL additions
should probably go.

llvm-svn: 214888