Patch generated local vars as vals if they are not mutated.

This is important for optimizations to be able to kick in with
enough accuracy. There are typically 2 kinds of such local vars
that are handled by this:
* ParamDefs of tail recursive methods
* Variables captured by lambdas or inner classes, and thus
  transformed to instances of IntRef/ObjectRef/etc. by
  lambdalift.

(not to mention vars declared by the programmer that they don't
mutate.)
Do you really want to bind `mutable` here?