[llvm] [NFC][TableGen] Code cleanup in CodeGenTarget.cpp (PR #125569)

[Rahul Joshi via llvm-commits]

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@@ -324,9 +325,8 @@ void CodeGenTarget::ComputeInstrsByEnum() const {
       });
 
   // Assign an enum value to each instruction according to the sorted order.
-  unsigned Num = 0;
-  for (const CodeGenInstruction *Inst : InstrsByEnum)
-    Inst->EnumVal = Num++;
+  for (const auto &[Idx, Inst] : enumerate(InstrsByEnum))
+    const_cast<CodeGenInstruction *>(Inst)->EnumVal = Idx;
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jurahul wrote:

Right, though mutable is universally true (i.e., any other code can change it without a const_cast) whereas with this you have to do something intentional to mutate the var, so safer. But I am fine going back to mutable. 

https://github.com/llvm/llvm-project/pull/125569