[CodeGen] Avoid repeated hash lookups (NFC) (#124506)

Author: kazutakahirata

llvm/lib/CodeGen/PrologEpilogInserter.cpp

@@ -208,8 +208,8 @@ static void stashEntryDbgValues(MachineBasicBlock &MBB,
   }
 
   // Remove stashed debug values from the block.
-  if (EntryDbgValues.count(&MBB))
-    for (auto *MI : EntryDbgValues[&MBB])
+  if (auto It = EntryDbgValues.find(&MBB); It != EntryDbgValues.end())
+    for (auto *MI : It->second)
       MI->removeFromParent();
 }