DebugInfo: Simplify scopes+subprogram handling since the subprogram<>cu link inversion

Previously this code was defensive to the situation in which the debug
info scopes would lead to a different subprogram from the subprogram in
the CU's subprogram list (this could've happened with linkonce
functions, etc as per the comment being removed). Since the CU<>SP link
reversal this is no longer possible.

llvm-svn: 303933