[mlir][ArmSME] Remove "pure" side-effect from 'get_tile_id' op to prevent CSE

[c-rhodes]

Summary

The 'get_tile_id' op is currently marked as pure so CSE thinks these ops
are equivalent and replaces with them a single one. This patch removes
pure to prevent this.

[awarzynski]

LGTM, thanks!

mlir/include/mlir/Dialect/ArmSME/IR/ArmSME.td

def CastVectorToTile : ArmSME_Op<"cast_vector_to_tile", [Pure, TileElementWidthMatchesTileID]> {
   }];
   let arguments = (ins SMETile:$vector);
   let results = (outs AnyTypeOf<[I8, I16, I32, I64, I128]>:$tile_id);
   let assemblyFormat =
     "$vector attr-dict `:` type($vector) `to` type($tile_id)";
   let hasCanonicalizeMethod = 1;
 }
 
-def GetTileID : ArmSME_Op<"get_tile_id", [Pure]> {
+def GetTileID : ArmSME_Op<"get_tile_id"> {
   let summary = "Returns an SME \"virtual tile\" id";
   let description = [{
     A `get_tile_id` operation returns a scalar integer representing an SME
     "virtual tile" id. The bitwidth of the scalar indicates the element
     bitwidth of the "virtual tile".
 
     The scope of a tile id is a function and cannot be passed or returned from
     functions.

mlir/test/Dialect/ArmSME/cse.mlir

+// RUN: mlir-opt -allow-unregistered-dialect %s -pass-pipeline='builtin.module(func.func(cse))' | FileCheck %s
+
+// This test is checking that CSE does not remove 'arm_sme.get_tile_id' ops as
+// duplicates.
+// CHECK-LABEL: @get_tile_id
+// CHECK: %[[TILE_ID_0:.*]] = arm_sme.get_tile_id : i32
+// CHECK: %[[TILE_ID_1:.*]] = arm_sme.get_tile_id : i32
+// CHECK: "prevent.dce"(%[[TILE_ID_0]]) : (i32) -> ()
+// CHECK: "prevent.dce"(%[[TILE_ID_1]]) : (i32) -> ()
+func.func @get_tile_id() {
+  %tile_id_1 = arm_sme.get_tile_id : i32
+  %tile_id_2 = arm_sme.get_tile_id : i32
+  "prevent.dce"(%tile_id_1) : (i32) -> ()
+  "prevent.dce"(%tile_id_2) : (i32) -> ()
+  return
+}