This refactors the {GetMemOp} function once again:
Instead of computing (mem_start + (offset_reg + offset_imm)), do compute
((mem_start + offset_imm) + offset_reg). This avoids an overflow in
(offset_reg + offset_imm) when using 32-bit computations, which hides
OOB memory accesses when relying on the trap handler.

As a nice side-effect, this change makes the whole method a lot nicer to
read.

We also need to change {StoreTaggedPointer} now, which was relying on the
inner working of {GetMemOp}. The new version makes the semantics more
transparent at the cost of repeating some logic from (the previous version
of) {GetMemOp}.

R=jkummerow@chromium.org

Bug: v8:11955, chromium:1227465, v8:11951
Change-Id: Ia068ca7c4f7db89b81529edd3438b0e4eee7d23d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3015566
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75693}