Changes:
- Don't use UNIMPLEMENTED where it can be run.
- Implement OpcodeLength for gc types.
- Remove unreachable code.
- Request the correct feature 'gc' for the respective opcodes and types.

Bug: 1079337, 1079450, 1081529, 1081011, 1080444

Change-Id: Ib081139ca805c6f5471a372f862638a3606254b1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2196302
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67755}