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.
No instruction sets actually have single instructions for materializing
vector constants. You always need to use a constant pool.
Cretonne doesn't have constant pools yet, but it will in the future, and
that is how vector constants should be represented.
