Introduce two new concepts:

1. Add support for defining Pattern's, which can match expressions when there
   is no instruction that directly implements something.  Instructions usually
   implicitly define patterns.
2. Add support for defining SDNodeXForm's, which are node transformations.
   This seperates the concept of a node xform out from the existing predicate
   support.

Using this new stuff, we add a few instruction patterns, one for testing, and
two for OR/XOR by an arbitrary immediate.

llvm-svn: 23286