[CodeGen] Remove atEnd method from defusechain iterators (#120610)

This was not used much and there are better ways of writing it.

llvm/include/llvm/CodeGen/MachineRegisterInfo.h

@@ -1095,9 +1095,6 @@ public:
       return !operator==(x);
     }
 
-    /// atEnd - return true if this iterator is equal to reg_end() on the value.
-    bool atEnd() const { return Op == nullptr; }
-
     // Iterator traversal: forward iteration only
     defusechain_iterator &operator++() {          // Preincrement
       assert(Op && "Cannot increment end iterator!");
@@ -1203,9 +1200,6 @@ public:
       return !operator==(x);
     }
 
-    /// atEnd - return true if this iterator is equal to reg_end() on the value.
-    bool atEnd() const { return Op == nullptr; }
-
     // Iterator traversal: forward iteration only
     defusechain_instr_iterator &operator++() {          // Preincrement
       assert(Op && "Cannot increment end iterator!");

llvm/lib/CodeGen/MachineRegisterInfo.cpp

@@ -407,9 +407,11 @@ void MachineRegisterInfo::replaceRegWith(Register FromReg, Register ToReg) {
 MachineInstr *MachineRegisterInfo::getVRegDef(Register Reg) const {
   // Since we are in SSA form, we can use the first definition.
   def_instr_iterator I = def_instr_begin(Reg);
-  assert((I.atEnd() || std::next(I) == def_instr_end()) &&
-         "getVRegDef assumes a single definition or no definition");
-  return !I.atEnd() ? &*I : nullptr;
+  if (I == def_instr_end())
+    return nullptr;
+  assert(std::next(I) == def_instr_end() &&
+         "getVRegDef assumes at most one definition");
+  return &*I;
 }
 
 /// getUniqueVRegDef - Return the unique machine instr that defines the

llvm/lib/CodeGen/MachineTraceMetrics.cpp

@@ -683,11 +683,10 @@ struct DataDep {
   DataDep(const MachineRegisterInfo *MRI, unsigned VirtReg, unsigned UseOp)
     : UseOp(UseOp) {
     assert(Register::isVirtualRegister(VirtReg));
-    MachineRegisterInfo::def_iterator DefI = MRI->def_begin(VirtReg);
-    assert(!DefI.atEnd() && "Register has no defs");
-    DefMI = DefI->getParent();
-    DefOp = DefI.getOperandNo();
-    assert((++DefI).atEnd() && "Register has multiple defs");
+    MachineOperand *DefMO = MRI->getOneDef(VirtReg);
+    assert(DefMO && "Register does not have unique def");
+    DefMI = DefMO->getParent();
+    DefOp = DefMO->getOperandNo();
   }
 };
 

llvm/lib/CodeGen/SwiftErrorValueTracking.cpp

@@ -259,7 +259,7 @@ void SwiftErrorValueTracking::propagateVRegs() {
   for (const auto &Use : VRegUpwardsUse) {
     const MachineBasicBlock *UseBB = Use.first.first;
     Register VReg = Use.second;
-    if (!MRI.def_begin(VReg).atEnd())
+    if (!MRI.def_empty(VReg))
       continue;
 
 #ifdef EXPENSIVE_CHECKS