X86: Custom lower v4i32 UMUL_LOHI into 2 pmuludqs.

Test will follow soon.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207314 91177308-0d34-0410-b5e6-96231b3b80d8
DAGCombiner: Turn divs of vector splats into vectorized multiplications.

Otherwise the legalizer would just scalarize everything. Support for
mulhi in the targets isn't that great yet so on most targets we get
exactly the same scalarized output. Add a test for x86 vector udiv.

I had to disable the mulhi nodes on ARM because there aren't any patterns
for it. As far as I know ARM has instructions for getting the high part of
a multiply so this should be fixed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207315 91177308-0d34-0410-b5e6-96231b3b80d8