[X86][XOP] Merge rotation opcodes with AVX512 equivalents. NFCI.

The XOP rotations act as ROTL with +ve values and ROTR with -ve values, which means that we can treat them all as ROTL with unsigned modulo. We already check that we're only trying to lower as ROTL for XOP rotations.

Differential Revision: https://reviews.llvm.org/D37949

llvm-svn: 314207