[llvm] [TargetLowering] Add a new function `getNullPtrValue` (PR #126665)

[Shilei Tian via llvm-commits]

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@@ -1807,8 +1807,7 @@ SDValue SelectionDAGBuilder::getValueImpl(const Value *V) {
 
     if (isa<ConstantPointerNull>(C)) {
       unsigned AS = V->getType()->getPointerAddressSpace();
-      return DAG.getConstant(0, getCurSDLoc(),
-                             TLI.getPointerTy(DAG.getDataLayout(), AS));
+      return TLI.getNullPtrValue(AS, getCurSDLoc(), DAG);
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shiltian wrote:

In addition, the [document](https://llvm.org/doxygen/classllvm_1_1ConstantPointerNull.html#details) suggests that it is "a constant pointer value that points to null". My reading is, it is a `nullptr`. I don't see where to solely treat it as 0.

https://github.com/llvm/llvm-project/pull/126665