RefineAsm: Better type inference.

Currently type inference only works for graph parameters, and does not support polymorphic parameter types.

**Improvement 1: Also infer return types**

This probably would require more complex global analysis. Not sure how to do this yet.

**Improvement 2: Polymorphic graph parameters**

This can be implemented by computing the intersection of all types used for each parameter in calls to a graph.

Start by locating RefineAsm's type-inference entry point and tracing how graph parameters are analyzed at call sites. The issue proposes two outcomes: infer return types and support polymorphic graph parameters by intersecting the types used for each parameter; it notes that return-type inference needs unresolved global analysis.

Written by the indexing model from the issue text.