Rip out X86-specific vector SDIV lowering, make the corresponding DAGCombiner transform work on vectors.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207316 91177308-0d34-0410-b5e6-96231b3b80d8
X86: Add patterns for MULHU/MULHS of v8i16 and v16i16.

This gets us pretty code for divs of i16 vectors. Turn the existing
intrinsics into the corresponding nodes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207317 91177308-0d34-0410-b5e6-96231b3b80d8