[MLIR, test] Fix use of undef FileCheck var

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.

Reviewed By: pifon2a

Differential Revision: https://reviews.llvm.org/D99958

Author: Thomas Preud'homme

mlir/test/Transforms/canonicalize.mlir

@@ -1129,8 +1129,8 @@ func @simple_clone_elimination() -> 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]]
 
 // -----