[AMDGPU] Switch to named simm16 in vscnt insertion

llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp

if (OldWaitcntInstr) {
           II->eraseFromParent();
           Modified = true;
         } else if (II->getOpcode() == AMDGPU::S_WAITCNT) {
           int64_t Imm = II->getOperand(0).getImm();
           ScoreBrackets.applyWaitcnt(AMDGPU::decodeWaitcnt(IV, Imm));
         } else {
           assert(II->getOpcode() == AMDGPU::S_WAITCNT_VSCNT);
           assert(II->getOperand(0).getReg() == AMDGPU::SGPR_NULL);
-          ScoreBrackets.applyWaitcnt(
-              AMDGPU::Waitcnt(~0u, ~0u, ~0u, II->getOperand(1).getImm()));
+          auto W = TII->getNamedOperand(*II, AMDGPU::OpName::simm16)->getImm();
+          ScoreBrackets.applyWaitcnt(AMDGPU::Waitcnt(~0u, ~0u, ~0u, W));
         }
       }
     }
     return Modified;
   }
 
   if (ForceEmitZeroWaitcnts)
     Wait = AMDGPU::Waitcnt::allZero(IV);
for (auto II = OldWaitcntInstr->getIterator(), NextI = std::next(II);
         }
         Wait.VmCnt = ~0u;
         Wait.LgkmCnt = ~0u;
         Wait.ExpCnt = ~0u;
       } else {
         assert(II->getOpcode() == AMDGPU::S_WAITCNT_VSCNT);
         assert(II->getOperand(0).getReg() == AMDGPU::SGPR_NULL);
 
-        unsigned ICnt = II->getOperand(1).getImm();
+        unsigned ICnt = TII->getNamedOperand(*II, AMDGPU::OpName::simm16)
+                        ->getImm();
         OldWait.VsCnt = std::min(OldWait.VsCnt, ICnt);
         if (!TrackedWaitcntSet.count(&*II))
           Wait.VsCnt = std::min(Wait.VsCnt, ICnt);
         if (Wait.VsCnt != ICnt) {
-          II->getOperand(1).setImm(Wait.VsCnt);
+          TII->getNamedOperand(*II, AMDGPU::OpName::simm16)->setImm(Wait.VsCnt);
           Modified = true;
         }
         Wait.VsCnt = ~0u;
       }
 
       LLVM_DEBUG(dbgs() << "generateWaitcntInstBefore\n"
                         << "Old Instr: " << MI
                         << "New Instr: " << *II << '\n');