LoadImmutable represents a load from a position in memory that is known
to be immutable, e.g. an immutable IsolateRoot or an immutable field of
a WasmInstanceObject. Because the returned value cannot change through
the execution of a function, LoadImmutable is a pure operator and does
not have effect or control edges.
This will allow more aggressive optimizations of loads of fields of
the Isolate and Instance that are known to be immutable.
Requires that the memory in question has been initialized at function
start even through inlining.

Note: We may reconsider this approach once we have escape analysis for
wasm, and replace it with immutable load/initialize operators that live
inside the effect chain and are less restriced.

Bug: v8:11510
Change-Id: I5e8e4f27d7008f39f01175ffa95a9c531ba63e66
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2775568Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73594}