Partially revert "[TableGen] Avoid repeated hash lookups (NFC) (#122586)"

This partially reverts commit 07ff786.

The hunk being reverted in this patch seems to break:

  tools/llvm-gsymutil/ARM_AArch64/macho-merged-funcs-dwarf.yaml

under LLVM_ENABLE_EXPENSIVE_CHECKS.

Author: kazutakahirata

llvm/utils/TableGen/Common/CodeGenSchedule.cpp

@@ -415,9 +415,9 @@ void CodeGenSchedModels::collectSTIPredicates() {
   for (const Record *R : Records.getAllDerivedDefinitions("STIPredicate")) {
     const Record *Decl = R->getValueAsDef("Declaration");
 
-    const auto [It, Inserted] =
-        Decl2Index.try_emplace(Decl, STIPredicates.size());
-    if (Inserted) {
+    const auto It = Decl2Index.find(Decl);
+    if (It == Decl2Index.end()) {
+      Decl2Index[Decl] = STIPredicates.size();
       STIPredicateFunction Predicate(Decl);
       Predicate.addDefinition(R);
       STIPredicates.emplace_back(std::move(Predicate));