[Coroutines] Avoid repeated hash lookups (NFC) (#127956)

Author: kazutakahirata

llvm/lib/Transforms/Coroutines/MaterializationUtils.cpp

@@ -293,7 +293,8 @@ void coro::doRematerializations(
     for (Instruction *U : E.second) {
       // Don't process a user twice (this can happen if the instruction uses
       // more than one rematerializable def)
-      if (AllRemats.count(U))
+      auto [It, Inserted] = AllRemats.try_emplace(U);
+      if (!Inserted)
         continue;
 
       // Constructor creates the whole RematGraph for the given Use
@@ -306,7 +307,7 @@ void coro::doRematerializations(
                       ++I) { (*I)->Node->dump(); } dbgs()
                  << "\n";);
 
-      AllRemats[U] = std::move(RematUPtr);
+      It->second = std::move(RematUPtr);
     }
   }