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.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@23286
91177308-0d34-0410-b5e6-
96231b3b80d8