Vector: Utilize range write barrier

Instead of emitting a barrier at each element construction, merely emit
a single write barrier at the end. This seems to be a small win on
throughput workloads.

Stylistic:
- Refactor blocks to check for Traits (e.g. kCanMoveWithMemcpy) first
  and then split the GC vs non-GC parts within those branches. This is
  subjectively easier to follow then a single if/else-if cascade.

Bug: 425736616
Change-Id: Ifb373b84304946c5ed2bf90c6e88af9c39699d23
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6642971
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1475461}