rtl-ssa: Fix null deref in first_any_insn_use

first_any_insn_use implicitly (but contrary to its documentation)
assumed that there was at least one use.

gcc/
	* rtl-ssa/member-fns.inl (first_any_insn_use): Handle null
	m_first_use.

gcc/rtl-ssa/member-fns.inl

@@ -215,7 +215,7 @@ set_info::last_nondebug_insn_use () const
 inline use_info *
 set_info::first_any_insn_use () const
 {
-  if (m_first_use->is_in_any_insn ())
+  if (m_first_use && m_first_use->is_in_any_insn ())
     return m_first_use;
   return nullptr;
 }