Better error messages when codegen-ing un-opted IR

#896 raises a good point about the error message being bad when you codegen IR that hasn't been optimized. Inlining is the main culprit- are there any other things that can cause codegen to mysteriously fail if you don't run opt.

See also #646 and #647.

An alternative to better error messages would be for codegen to run something like `-O0` passes in codegen itself, which might just be inlining functions (and a very cheap no-op if opt has already run).