[SelectionDAG] Support known true/false SimplifySetCC cases for comparing...

[SelectionDAG] Support known true/false SimplifySetCC cases for comparing against vector splats of constants.

This is split off from D42948 and includes just the cases that constant fold to true or false. It also includes some refactoring to keep predicate checks together.

This supports things like

(setcc uge X, 0) -> true

Differential Revision: https://reviews.llvm.org/D43489

llvm-svn: 325627