Changes:
- Add current global index argument to consume_init_expr.
- Inline DecodeGlobalInModule. Move the check for undefined global
  indexes into into consume_init_expr. Note: This fixes a bug where the
  index wasn't checked for nested global.get.
- Under --experimental-wasm-gc, allow global initializers to reference
  already defined globals in the same module.
- Rename ModuleDecoderImpl::DecodeInitExpr -> DecodeInitExprForTesting.
  Remove redundant "start" argument.
- Add tests for global initializers. Remove a redundant test.

Bug: v8:7748
Change-Id: Ieb4a768f8cfdd423e5f439bb3467700068f240b7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2428596
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70181}