Fix problem with inserting FunctionPasses that depend on ImmutablePasses

(e.g., LICM) into FunctionPassManagers. The problem is that we were
using a C-style cast to cast required analysis passes to PassClass*, but
if it's a FunctionPassManager, and the required analysis pass is an
ImmutablePass, the types aren't really compatible, so the C-style cast
causes a crash.

llvm-svn: 15140