[globalopt] Don't emit DWARF fragments for members

of a struct that cover the whole struct

This can happen when the rest of the
members of are zero length. Following
the same pattern applied to the SROA
pass in:
d7f6f163

Fixes: https://bugs.llvm.org/show_bug.cgi?id=45335

Differential Revision: https://reviews.llvm.org/D78720