Design doc: https://bit.ly/3xPxWUe

This CL introduces two main changes:
- Initializer expressions are now decoded by WasmFullDecoder. With
  wasm-gc, initializer expressions are no longer just constants, and
  require complex decoding (including stack tracking). This resulted in
  extensive code duplication.
- Initializer expressions are not stored explicitly by module-decoder as
  an AST (WasmInitExpr), but rather as a WireBytesRef, and are decoded
  again during module instantiation. This should reduce memory
  consumption for globals and other module elements with initializer
  expressions (which has been observed in the 40MB range in some
  real-world benchmarks.

Summary of changes:
- Add a static parameter {kFunctionBody, kInitExpression} to the
  WasmDecoder. Use it to specialize validation to function bodies/init.
  expressions.
- Introduce a new Interface for the WasmFullDecoder for init.
  expressions.
- Differentiate between constant and non-constant opcodes in
  WasmFullDecoder.
- Change representation of init. expressions in WasmModule to
  WireBytesRef.
- Reimplement EvaluateInitExpression in module-instantiate to re-decode
  initializer expressions.
- Remove some now-invalid module decoder tests.

Pending changes:
- Also refactor initializer expressions for element segment entries.
- Reintroduce deleted tests.

Bug: v8:11895
Change-Id: I76512bfe1386c8338667d30fa6db93880a1e4b42
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2972910Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75476}