TableGen: implement predicate tree and basic simplification

A recent change in TableGen definitions allowed arbitrary AND/OR predicate
compositions at the cost of removing known-true predicate simplification.
Introduce a more advanced simplification mechanism instead.

In particular, instead of folding predicate C++ expressions directly in
TableGen, keep them as is and build a predicate tree in TableGen C++ library.
The predicate expression-substitution mechanism, necessary to implement complex
predicates for nested classes such as `ContainerType`, is replaced by a
dedicated predicate.  This predicate appears in the predicate tree and can be
used for tree matching and separation.  More specifically, subtrees defined
below such predicate may be subject to different transformations than those
that appear above.  For example, a subtree known to be true above the
substitution predicate is not necessarily true below it.

Use the predicate tree structure to eliminate known-true and known-false
predicates before code emission, as well as to collapse AND and OR predicates
if their value can be deduced based on the value of one child.

PiperOrigin-RevId: 229605997