[llvm][NVPTX] Inform that 'DYNAMIC_STACKALLOC' is unsupported

Catch unsupported path early up, and emit error with information.

Motivated by the following threads:
* https://discourse.llvm.org/t/nvptx-problems-with-dynamic-alloca/70745
* #64017

Author: JOE1994

llvm/lib/Target/NVPTX/NVPTXISelLowering.cpp

@@ -639,6 +639,11 @@ NVPTXTargetLowering::NVPTXTargetLowering(const NVPTXTargetMachine &TM,
   setOperationAction(ISD::ConstantFP, MVT::f16, Legal);
   setOperationAction(ISD::ConstantFP, MVT::bf16, Legal);
 
+  // Lowering of DYNAMIC_STACKALLOC is unsupported.
+  // Custom lower to produce an error.
+  setOperationAction(ISD::DYNAMIC_STACKALLOC, MVT::i32, Custom);
+  setOperationAction(ISD::DYNAMIC_STACKALLOC, MVT::i64, Custom);
+
   // TRAP can be lowered to PTX trap
   setOperationAction(ISD::TRAP, MVT::Other, Legal);
 
@@ -2652,6 +2657,9 @@ NVPTXTargetLowering::LowerOperation(SDValue Op, SelectionDAG &DAG) const {
   case ISD::SREM:
   case ISD::UREM:
     return LowerVectorArith(Op, DAG);
+  case ISD::DYNAMIC_STACKALLOC:
+    report_fatal_error(
+        "Dynamic stack allocation is yet unsupported by NVPTX backend.");
   default:
     llvm_unreachable("Custom lowering not defined for operation");
   }