Design doc: bit.ly/36MfD6Y

We introduce simplified operators LoadImmutableFromObject and
InitializeImmutableInObject. These are lowered to Loads and Stores like
LoadFromObject and StoreToObject.
We split CsaLoadElimination::AbstractState in two HalfStates,
which represent the mutable and immutable component of the state.
Immutable operators in the effect chain modify the immutable half-state,
and plain operators modify the mutable half-state. The immutable part is
maintained through write effects and loop headers. Immutable
initializations do not lookup and kill previous overlapping stores,
assuming each offset cannot be initialized more than once.

Bug: v8:11510

Change-Id: I0f5feca3354fdd3bdc1f511cc5214ec51e1407ad
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3268728Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78325}