Constrain mvars under compare and branch with built-in predicates

Constrain mvars under compare and branch with built-in predicates

	* lisp/emacs-lisp/comp.el (comp-emit-assume): Update.
	(comp-known-predicate-p): New function.
	(comp-add-cond-cstrs): Extend to pattern match predicate calls.
	* lisp/emacs-lisp/comp-cstr.el (comp-cstr-null-p)
	(comp-pred-to-cstr): New function.
	* test/src/comp-tests.el (comp-tests-type-spec-tests): Add a
	number of tests and fix comments.