Compile time tweak for libcall lookup

If we have a large module which is mostly intrinsics, we hammer the lib call lookup path from CodeGenPrepare.  Adding a fastpath reduces compile by 15% for one such example.

The problem is really more general than intrinsics - a module with lots of non-intrinsics non-libcall calls has the same problem - but we might as well avoid an easy case quickly.

llvm-svn: 360391