The meta language patterns sometimes need to refer to specific values of
enumerated immediate operands. The dot syntax provides a namespaced,
typed way of doing that: icmp(intcc.ult, a, x).
Add an ast.Enumerator class for representing this kind of AST leaf node.
Add value definitions for the intcc and floatcc immediate operand kinds.
The carry and borrow values are boolean, so we have to convert them to
an integer type with bint(c) before we can add them to the result.
Also tweak the default legalizer action for unsupported types: Only
attempt a narrowing pattern for lane types > 32 bits.
This was found by @angusholder's new type checks in the verifier.
