Fix an instance of -Wdeprecated-copy.

Bug: chromium:1235909
Change-Id: I1eb320bafb276ff093a4f0d6b184fe77607c4678
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3116887
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Nico Hartmann <nicohartmann@chromium.org>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76479}

src/codegen/x64/assembler-x64.h

@@ -235,6 +235,7 @@ class V8_EXPORT_PRIVATE Operand {
   }
 
   Operand(const Operand&) V8_NOEXCEPT = default;
+  Operand& operator=(const Operand&) V8_NOEXCEPT = default;
 
   const Data& data() const { return data_; }