[CodeGen] Use DenseMap::operator[] (NFC) (#108489)

Once we modernize CopyInfo with default member initializations,

  Copies.insert({Unit, ...})

becomes equivalent to:

  Copies.try_emplace(Unit)

which we can simplify further down to Copies[Unit].

Author: kazutakahirata

llvm/lib/CodeGen/MachineCopyPropagation.cpp

@@ -108,9 +108,10 @@ static std::optional<DestSourcePair> isCopyInstr(const MachineInstr &MI,
 
 class CopyTracker {
   struct CopyInfo {
-    MachineInstr *MI, *LastSeenUseInCopy;
+    MachineInstr *MI = nullptr;
+    MachineInstr *LastSeenUseInCopy = nullptr;
     SmallVector<MCRegister, 4> DefRegs;
-    bool Avail;
+    bool Avail = false;
   };
 
   DenseMap<MCRegUnit, CopyInfo> Copies;
@@ -240,8 +241,7 @@ class CopyTracker {
     // Remember source that's copied to Def. Once it's clobbered, then
     // it's no longer available for copy propagation.
     for (MCRegUnit Unit : TRI.regunits(Src)) {
-      auto I = Copies.insert({Unit, {nullptr, nullptr, {}, false}});
-      auto &Copy = I.first->second;
+      auto &Copy = Copies[Unit];
       if (!is_contained(Copy.DefRegs, Def))
         Copy.DefRegs.push_back(Def);
       Copy.LastSeenUseInCopy = MI;