IRGen: Move vtable load after argument evaluation.

This change reduces the live range of the loaded function pointer,
resulting in a slight code size decrease (~10KB in clang), and also
improves the security of CFI for virtual calls by making it less
likely that the function pointer will be spilled, and ensuring that
it is not spilled across a function call boundary.

Fixes PR35353.

Differential Revision: https://reviews.llvm.org/D42725

llvm-svn: 324286