[BACKEND] hint to LLVM that we can bound threadIdx.x (#7249) (#4737)

877cf7a relanded most of
e7cef2e, but missed one change.

Co-authored-by: apgoucher <[email protected]>

Author: whitneywhtsang

test/Conversion/reduce_to_llvm.mlir

@@ -25,7 +25,7 @@ tt.func private @reduce_linear_layout(%arg0: tensor<32x16xi32, #linear>) -> tens
   // is not needed.
 
   // Reduce within threads
-  // CHECK-NEXT: [[SUM0:%.*]] = add i32 [[SRC0]], [[SRC2]]
+  // CHECK: [[SUM0:%.*]] = add i32 [[SRC0]], [[SRC2]]
   // CHECK-NEXT: [[SUM1:%.*]] = add i32 [[SRC1]], [[SRC3]]
 
   // Reduce within warp.