X86: Add a couple of target-specific dag combines that turn VSELECTS into psubus...

X86: Add a couple of target-specific dag combines that turn VSELECTS into psubus if possible.

We match the pattern "x >= y ? x-y : 0" into "subus x, y" and two special cases
if y is a constant. DAGCombiner canonicalizes those so we first have to undo the
canonicalization for those cases. The pattern occurs in gzip when the loop
vectorizer is enabled. Part of PR14613.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@170273 91177308-0d34-0410-b5e6-96231b3b80d8