[ARM] Simplify code with std::map::operator[] (NFC) (llvm#112159)

Author: kazutakahirata

llvm/lib/Target/ARM/ARMBaseInstrInfo.cpp

@@ -6868,15 +6868,13 @@ bool ARMPipelinerLoopInfo::tooMuchRegisterPressure(SwingSchedulerDAG &SSD,
       if (MI->isPHI() && S.getKind() == SDep::Anti) {
         Register Reg = S.getReg();
         if (Reg.isVirtual())
-          CrossIterationNeeds.insert(std::make_pair(Reg.id(), IterNeed()))
-              .first->second.set(0);
+          CrossIterationNeeds[Reg.id()].set(0);
       } else if (S.isAssignedRegDep()) {
         int OStg = SMS.stageScheduled(S.getSUnit());
         if (OStg >= 0 && OStg != Stg) {
           Register Reg = S.getReg();
           if (Reg.isVirtual())
-            CrossIterationNeeds.insert(std::make_pair(Reg.id(), IterNeed()))
-                .first->second |= ((1 << (OStg - Stg)) - 1);
+            CrossIterationNeeds[Reg.id()] |= ((1 << (OStg - Stg)) - 1);
         }
       }
   }