Make ConvertableToGEP handle cases where the user is indexing into the

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@1931 91177308-0d34-0410-b5e6-96231b3b80d8