[InstCombine] Teach how to convert SSSE3/AVX2 byte shuffles to builtin...

[InstCombine] Teach how to convert SSSE3/AVX2 byte shuffles to builtin shuffles if the shuffle mask is constant.

This patch teaches InstCombiner how to convert a SSSE3/AVX2 byte shuffle to a
builtin shuffle if the mask is constant.

Converting byte shuffle intrinsic calls to builtin shuffles can help finding
more opportunities for combining shuffles later on in selection dag.

We may end up with byte shuffles with constant masks as the result of inlining.

Differential Revision: http://reviews.llvm.org/D13252

llvm-svn: 248913