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

[Piotr Fusik via llvm-commits]

================
@@ -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);
       else
         return SDValue();
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pfusik wrote:

Do you mean:

```cpp
auto OptBits = Elt->bitcastToAPInt();
if (!OptBits)
  return SDValue();
APInt Bits = *OptBits;
```
?

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