Several small optimisations for TranslationArray:

  a) Store opcodes and register codes as unsigned values (no need to
     shift in the sign bit when encoding/decoding). Note that skips over
     register codes will decode them as if they were signed -- this is
     ok since we don't use the skipped value.

  b) Use the static knowledge that opcodes and register codes need 7
     bits to avoid the VLQEncode loop when building (still use a
     VLQDecode when decoding since decode time matters less).

  c) Add a special opcode for "optimized out", instead of using a
     literal, since this will be a common case.

Change-Id: I9758e5b889ecc3f1a3fa4d840867f2a3d481e75f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3812040
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Nico Hartmann <nicohartmann@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82227}