Fix the order of directives and the target string

[Chia-hungDuan]

Summary

In the original structure, it will try to match CHECK-LABEL first then see if
the subsequent doesn't have the target strings. This is not what we are
expected. We are expecting the two functions which will be deleted should be
matched before CHECK-LABEL. Also fixed the function names.

[jpienaar]

Good catch

mlir/test/mlir-reduce/dce-test.mlir

 // UNSUPPORTED: system-windows
 // RUN: mlir-reduce %s -test %S/failure-test.sh -pass-test DCE | FileCheck %s
 // This input should be reduced by the pass pipeline so that only
 // the @simple1 function remains as the other functions should be
 // removed by the dead code elimination pass.
-// CHECK-LABEL: func @simple1(%arg0: i1, %arg1: memref<2xf32>, %arg2: memref<2xf32>) {
 
-// CHECK-NOT: func @dead_nested_function
+// CHECK-NOT: func private @dead_private_function
 func private @dead_private_function()
 
-// CHECK-NOT: func @dead_nested_function
+// CHECK-NOT: func nested @dead_nested_function
 func nested @dead_nested_function()
 
+// CHECK-LABEL: func @simple1(%arg0: i1, %arg1: memref<2xf32>, %arg2: memref<2xf32>) {
 func @simple1(%arg0: i1, %arg1: memref<2xf32>, %arg2: memref<2xf32>) {
   "test.crashOp" () : () -> ()
   return
 }