cppgc: Avoid compression/decompression in Member operators/ctors

1) In copy/move ctors and operator=() we can just copy raw compressed
   value;
2) For null check we don't need to decompress the value;
3) Same for operator==().
4) Hashing can also be optimized in a followup.

Bug: chromium:1325007
Change-Id: Ic1bf2c5049802c078b3e0121dcbe62d9ecea83b3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3647359Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80571}