Motivation: With rtt.sub now allowed in constant expressions, we have
to generalize WasmInitExpr to be able to handle expressions with
operands. This CL prepares the ground for this change and adds no
functionality.

Changes:
- ValueType::heap_representation and HeapType::representation now
  return HeapType::Representation.
- Add ValueType::is_rtt().
- WasmInitExpr:
  - Make kind private. Rename val -> operator, make it private. Add
    accessors.
  - Rename kGlobalIndex -> kGlobalGet.
  - Squash global_index and function_index into index.
  - Add heap_type Immediate. Use it for RefNullConst. TypeOf in
    module-decoder.cc can now fully determine the type of a
    WasmInitExpr.
  - Add class constructors/static method constructors for each Operator
    kind.
  - Delete copy constructor. WasmInitExpr will use std::unique_ptr for
    its operands.
- consume_init_expr now uses a stack.
- A few minor improvements.

Bug: v8:7748
Change-Id: I3ba3ee7ac2d6bc58e887790c37110ceb80658985
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2284483
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68759}