[opaque pointer type] Bitcode support for explicit type parameter on the gep operator

This happened to be fairly easy to support backwards compatibility based
on the number of operands (old format had an even number, new format has
one more operand so an odd number).

test/Bitcode/old-aliases.ll already appears to test old gep operators
(if I remove the backwards compatibility in the BitcodeReader, this and
another test fail) so I'm not adding extra test coverage here.

llvm-svn: 232216