Cast vtable address points to i32 (...)** to enable more globalopt

We currently use i32 (...)** as the type of the vptr field in the LLVM
struct type. LLVM's GlobalOpt prefers any bitcasts to be on the side of
the data being stored rather than on the pointer being stored to.

Reviewers: majnemer

Differential Revision: http://reviews.llvm.org/D5916

llvm-svn: 223267