[SelectionDAG] Support some SimplifySetCC cases for comparing against vector splats of constants.

This supports things like

(setcc ugt X, 0) -> (setcc ne X, 0)

I've restricted to only make changes to vectors before legalize ops because I doubt all targets have accurate condition code legality information for vectors given how little we did before.

Differential Revision: https://reviews.llvm.org/D42948

llvm-svn: 326495