[MLIR, test] Fix use of undef FileCheck var

[thopre]

Summary

MLIR test Transforms/canonicalize.mlir tries to check for the absence of
a sequence of instructions with several CHECK-NOT with one of those
directives using a variable defined in another. However CHECK-NOT are
checked independently so that is using a variable defined in a pattern
that should not occur in the input.

This commit removes the dependency between those CHECK-NOT by replacing
occurences of variables by the regex that were used to define them.

[pifon2a]

thanks

mlir/test/Transforms/canonicalize.mlir

 // CHECK-LABEL: func @simple_clone_elimination
 func @simple_clone_elimination() -> memref<5xf32> {
   %ret = memref.alloc() : memref<5xf32>
   %temp = memref.clone %ret : memref<5xf32> to memref<5xf32>
   memref.dealloc %temp : memref<5xf32>
   return %ret : memref<5xf32>
 }
 // CHECK-NEXT: %[[ret:.*]] = memref.alloc()
-// CHECK-NOT: %[[temp:.*]] = memref.clone
-// CHECK-NOT: memref.dealloc %[[temp]]
+// CHECK-NOT: %{{.*}} = memref.clone
+// CHECK-NOT: memref.dealloc %{{.*}}
 // CHECK: return %[[ret]]
 
 // -----
 
 // CHECK-LABEL: func @clone_loop_alloc
 func @clone_loop_alloc(%arg0: index, %arg1: index, %arg2: index, %arg3: memref<2xf32>, %arg4: memref<2xf32>) {
   %0 = memref.alloc() : memref<2xf32>
   memref.dealloc %0 : memref<2xf32>