Some SELECT_CC cleanups:

Some SELECT_CC cleanups:
1. move assertions for node creation to getNode()
2. legalize the values returned in ExpandOp immediately
3. Move select_cc optimizations from SELECT's getNode() to SELECT_CC's,
   allowing them to be cleaned up significantly.

This paves the way to pick up additional optimizations on SELECT_CC, such
as sum-of-absolute-differences.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@22757 91177308-0d34-0410-b5e6-96231b3b80d8