Rework our internal representation of node predicates to expose more

Rework our internal representation of node predicates to expose more
structure and fix some fixmes.  We now have a TreePredicateFn class
that handles all of the decoding of these things.  This is an internal
cleanup that has no impact on the code generated by tblgen.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@129670 91177308-0d34-0410-b5e6-96231b3b80d8