In wasm-compiler, nodes that reference the Isolate and Instance should
depend on whether the current frame JS or Wasm. This was implemented
with mutable fields caching nodes in WasmGraphBuilder and its
subclasses, but was not done consistently.
This CL removes all mutability and replaces it with an additional field
isolate_ in WasmGraphBuilder, which is only set in the constructor of
the child class WasmWrapperGraphBuilder.

Additional changes:
- Remove three almost-unused utilities from WasmGraphAssembler.
- Reduce duplicate BuildImportCall with int index argument to the one
  with Node* index argument (the additional operations should be
  optimized away).

Bug: v8:11510
Change-Id: I2bb44a617210916edf671c238402727e04bdc38f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2739976Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73444}