[Ur] Let inlining interfering with staging

[Edward Z. Yang]

Excerpts from Adam Chlipala's message of Wed Sep 19 15:05:13 -0700 2012:
> I've added an embarrassing "fix" for this problem: a certain inlining 
> optimization skips firing if the type of the variable indicates it could 
> have been returned by an FFI function. :D

Tests good!

> Clearly the next redesign of Ur/Web should have a more principled, 
> type-based approach to controlling where different categories of effects 
> are legal.

Hear hear.