Make ConvertableToGEP handle cases where the user is indexing into the

first element of a structure type.  Before this would not be handled because
getStructOffset would either stop immediately (because StopEarly was true
and Offset = 0), or blast past the level we wanted.

Now ConvertableToGEP steps down through the type one level at a time, checking
the Offset and Scale conditions at each step

llvm-svn: 1931