[opaque pointer types] Cleanup CGBuilder's Create*GEP.

The various EltSize, Offset, DataLayout, and StructLayout arguments
are all computable from the Address's element type and the DataLayout
which the CGBuilder already has access to.

After having previously asserted that the computed values are the same
as those passed in, now remove the redundant arguments from
CGBuilder's Create*GEP functions.

Differential Revision: https://reviews.llvm.org/D57767

llvm-svn: 353629