Implement the instruction ref.as_non_null, as per the wasm gc extension.

Changes:
- Add the respective wasm opcode, move some asmjs opcodes around.
- Add a new type of wasm trap, IllegalCast.
- Modify wasm decoding and compilation pipeline.
- Add a minimal test.
- In wasm-compiler, generalize Unreachable to Trap.
- Optimize struct.get and struct.set for non-null types.

Bug: v8:7748
Change-Id: If2f794306c7cbfabc06e4f64988132346085d6dd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2187616
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67705}