Instead of searching the entire type graph for a type to determine if it

contains the type we are looking for, just search the immediately used types.
We can only do this because we keep the "current" type in the nesting level
as we decrement upreferences.

This change speeds up the testcase in PR224 from 50.4s to 22.08s, not
too shabby.

llvm-svn: 11221