[MLIR, test] Fix use of undef FileCheck var

[thopre]

Summary

MLIR test Dialect/Linalg/tile-indexed-generic.mlir has a CHECK-NOT
directive referring to a variable only defined in a CHECK directive with
a different prefix, and thus undefined in the CHECK-NOT.

This commit removes the variable reference altogether to error on any
content it might have.

mlir/test/Dialect/Linalg/tile-indexed-generic.mlir

 // TILE-25n0:   linalg.indexed_generic
 // TILE-25n0:   ^bb0(%[[I:.*]]: index, %[[IN:.*]]: f32, %[[OUT:.*]]: f32)
 // TILE-25n0:     %[[NEW_I:.*]] = addi %[[I]], %[[J]] : index
 // TILE-25n0:     %[[NEW_I_INT:.*]] = index_cast %[[NEW_I]] : index to i32
 // TILE-25n0:     %[[NEW_I_FLOAT:.*]] = sitofp %[[NEW_I_INT]] : i32 to f32
 // TILE-25n0:     %[[OUT:.*]] = addf %[[IN]], %[[NEW_I_FLOAT]] : f32
 
 // TILE-0n25-LABEL: func @indexed_generic_vector
-// TILE-0n25-NOT: scf.for %[[J:.*]] = {{.*}} step %[[C25]]
+// TILE-0n25-NOT: scf.for %[[J:.*]] = {{.*}} step %
 // TILE-0n25: linalg.indexed_generic
 
 #combined_indices_trait = {
   args_in = 1,
   args_out = 1,
   indexing_maps = [
     affine_map<(i, j) -> (j, i + j)>,
     affine_map<(i, j) -> (i, j)>