[opt] Inline a trivial lookup function into the header.

This function is actually *very* hot. It is hard to see currently
because the call graph is very recursive, but I'm working to remove that
and when I do this function becomes significantly higher on the profile
(up to 5%!) and so worth avoiding the call overhead.

No specific perf gain I can measure yet (below the noise), but likely to
have more impact as we stop cluttering the call graph.

Differential Revision: http://reviews.llvm.org/D10788

llvm-svn: 240873