When filling stack slots, the start offset can be too large to fit into
the immediate of a str instruction (which is used to handle remainders
after stp). For example, a function with 32 i64 params will require 256
bytes reserved for the params, so the offset starts at 256 + 16
(instance) = 272. This does not fit into a int9, so we hit an
UNREACHABLE case when emitting str.

The fix here checks that start can fit in an unscaled immediate, and if
it doesn't fallback to the general case. We could use the Str
from macro-asesmbler, but that uses another instruction, so we are not
saving anything.

A check for IsImmLSUnscaled(-start-12) is sufficient because 12 is the
largest possible value for remainder. So if -start-12 fits, everything
else will fit.

Bug: v8:10645
Change-Id: I1c415499ada3a807d5f3889f091150bfefdf471d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2267369
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68594}