Add support for having multiple predicates on a TreePatternNode.

Add support for having multiple predicates on a TreePatternNode.
This will allow predicates to be composed, which will allow the
predicate definitions to become less redundant, and eventually
will allow DAGISelEmitter.cpp to emit less redundant code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@57562 91177308-0d34-0410-b5e6-96231b3b80d8