[wasm] Do not use WasmInitExpr for element segments

Changes:
- Use a lightweight WasmElemSegment::Entry struct to store element
  segment entries in a WasmModule.
- Also, restructure LoadElemSegmentImpl to handle all types of
  global.get entries correctly.
- Simplify InitializeIndirectFunctionTables and make it handle all types
  of entries correctly.
- In the above two cases, reject WasmJSFunctions for now.

Bug: v8:11895
Change-Id: Ie714f8c7f1af8959486138d2ad49bc622a89276d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2991248


Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75513}