[llvm] [NVPTX] Add some basic folds for ADDRSPACECAST (PR #129157)

[Artem Belevich via llvm-commits]

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@@ -5209,6 +5209,26 @@ PerformBUILD_VECTORCombine(SDNode *N, TargetLowering::DAGCombinerInfo &DCI) {
   return DAG.getNode(ISD::BITCAST, DL, VT, PRMT);
 }
 
+static SDValue combineADDRSPACECAST(SDNode *N,
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Artem-B wrote:

This will potentially hide invalid casts between specific AS'es.
E.g. `ASC(shared->local->shared)` or `ASC(shared->local->generic`) will all of a sudden start working.

While we may be allowed to do so, because it's the input that's invalid, hiding such errors may not be the best option.
I'd prefer to narrow the folding to apply only to the valid casts, and leave invalid ones as is -- they will be handled/or not when we try lowering individual casts and, at least, will still be present in the DAG in their original form.

https://github.com/llvm/llvm-project/pull/129157