Loading from/storing to the same field with incompatible mutabilities
is possible in unreachable code, specifically when a value is cast to
two different types with incompatible mutability for the same field
offset. Therefore, we allow this pattern in CsaLoadElimination.
When we detect it, we emit an Unreachable node to immediately crash the
program in case this unreachable code is somehow executed.

Bug: v8:7748, v8:12874
Change-Id: Ieb359d3e1b9f7bc4a91c556af2bba0507526d20e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3644806
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80587}