Ignore allocation bytes in one more mir-opt test

Following on PR #126502, add `rustc -Zdump-mir-exclude-alloc-bytes`
to tests/mir-opt/dataflow-const-prop/aggregate_copy.rs as well
to skip writing allocation bytes in MIR dumps.

Fixes #126261

tests/mir-opt/dataflow-const-prop/aggregate_copy.foo.DataflowConstProp.diff

@@ -57,9 +57,7 @@
           StorageDead(_1);
           return;
       }
-+ }
-+ 
-+ ALLOC0 (size: 8, align: 4) {
-+     05 00 00 00 03 00 00 00                         │ ........
   }
++ 
++ ALLOC0 (size: 8, align: 4) { .. }
 

tests/mir-opt/dataflow-const-prop/aggregate_copy.rs

@@ -1,6 +1,7 @@
 //! Verify that we manage to propagate the value of aggregate `a` even without directly mentioning
 //! the contained scalars.
 //@ test-mir-pass: DataflowConstProp
+//@ compile-flags: -Zdump-mir-exclude-alloc-bytes
 
 const Foo: (u32, u32) = (5, 3);