rtl generation & predicates

[Joern Rennecke]

> More to the point, the gen_* functions assume the caller has already called the
> appropriate predicate functions, and adpopted alternative codegen strategies if
> the predicates failed, such as copying the argument to a pseudo registers.

Well, it is also possible that you have multiple patterns that differ only
in their predicates and output templates.  Then you can pick one gen_*
function and use it for any of these similar patterns.