llvm-cov: Only combine segments if they overlap exactly

If two coverage segments cover the same area we need to combine them,
as per r218432. OTOH, just because they start at the same place
doesn't mean they cover the same area. This fixes the check to be more
exact about this.

This is pretty hard to test right now. The frontend doesn't currently
emit regions that start at the same place but don't overlap, but some
upcoming work changes this.

llvm-svn: 227017