[ORC] Fix handling of casts in llvm.global_ctors.

Removes a bogus dyn_cast_or_null that was breaking cast-expression handling when
parsing llvm.global_ctors.

The intent of this code was to identify Functions nested within cast
expressions, but the offending dyn_cast_or_null was actually blocking that:
Since a function is not a cast expression, we would set FuncC to null and break
the loop without finding the Function. The cast was not necessary either:
Functions are already Constants, and we didn't need to do anything
ConstantExpr-specific with FuncC, so we could just drop the cast.

Thanks to Jonas Hahnfeld for tracking this down.

http://llvm.org/PR54797