[ByteCode] Avoid repeated hash lookups (NFC) (llvm#132141)

Author: kazutakahirata

clang/lib/AST/ByteCode/Program.cpp

@@ -213,12 +213,12 @@ std::optional<unsigned> Program::createGlobal(const ValueDecl *VD,
   if (auto Idx =
           createGlobal(VD, VD->getType(), IsStatic, IsExtern, IsWeak, Init)) {
     for (const Decl *P = VD; P; P = P->getPreviousDecl()) {
+      unsigned &PIdx = GlobalIndices[P];
       if (P != VD) {
-        unsigned PIdx = GlobalIndices[P];
         if (Globals[PIdx]->block()->isExtern())
           Globals[PIdx] = Globals[*Idx];
       }
-      GlobalIndices[P] = *Idx;
+      PIdx = *Idx;
     }
     return *Idx;
   }