[llvm] [SelectionDAG] Add an ISD node for for get.active.lane.mask (PR #139084)

[Paul Walker via llvm-commits]

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@@ -1524,6 +1524,12 @@ enum NodeType {
   // Operands: Mask
   VECTOR_FIND_LAST_ACTIVE,
 
+  // Creates a mask representing active and inactive
+  // vector lanes, active while base < trip count.
+  // Operands: Base, Trip Count
+  // Output: Mask
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paulwalker-arm wrote:

This could do with more details, for example, to reference the internal addition cannot overflow or perhaps reference the LLVM intrinsic's documentation and then just call out any differences.  For example, the ISD node supports different result types that follow the same rules as `ISD::SETCC`, specifically "If the result value type is not i1 then the high bits conform to getBooleanContents".

https://github.com/llvm/llvm-project/pull/139084