[llvm] [SelectionDAG][NFC] Refactor duplicate code into SDNode::bitcastToAPInt() (PR #127503)

[Piotr Fusik via llvm-commits]

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@@ -27420,10 +27420,8 @@ SDValue DAGCombiner::XformToShuffleWithZero(SDNode *N) {
       }
 
       APInt Bits;
-      if (auto *Cst = dyn_cast<ConstantSDNode>(Elt))
-        Bits = Cst->getAPIntValue();
-      else if (auto *CstFP = dyn_cast<ConstantFPSDNode>(Elt))
-        Bits = CstFP->getValueAPF().bitcastToAPInt();
+      if (auto OptBits = Elt->bitcastToAPInt())
+        Bits = *std::move(OptBits);
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pfusik wrote:

I don't think this makes much difference. `operator*` has [an r-value overload](https://en.cppreference.com/w/cpp/utility/optional/operator*).

But:
1. [cppreference](https://en.cppreference.com/w/cpp/utility/optional/operator*) has an `*std::move(opt2)` example.
2. I read `std::move` as "I will no longer use this variable", so it makes more sense to me on a variable than on the result of `operator*`.


https://github.com/llvm/llvm-project/pull/127503