As Wasm code increasingly needs to deal with various HeapObject
subtypes, the WasmGraphAssembler is a good place to consolidate
common helper methods like LoadMap or LoadFixedArrayElement.
This is clearly inspired by the CodeStubAssembler, and there is
clearly room for much more refactoring in this direction.

This CL does not intend to change any functionality.

Bug: v8:11074
Change-Id: I1b768c5791bde7041bc9f41a3069afb1844cdb46
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2532083Reviewed-by: Manos Koukoutos <manoskouk@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71201}