[llvm] [SelectionDAG] Move SelectionDAG::getAllOnesConstant out of line. NFC (PR #102995)

[Matt Arsenault via llvm-commits]

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@@ -675,10 +675,7 @@ class SelectionDAG {
                       bool isTarget = false, bool isOpaque = false);
 
   SDValue getAllOnesConstant(const SDLoc &DL, EVT VT, bool IsTarget = false,
-                             bool IsOpaque = false) {
-    return getConstant(APInt::getAllOnes(VT.getScalarSizeInBits()), DL, VT,
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arsenm wrote:

But that sounds like a bug, generally constants are sign extended in other contexts in llvm (e.g. in MachineOperand) 

https://github.com/llvm/llvm-project/pull/102995