The if above asks for field_representation to have Compressed or Any
Machine Representation. Therefore, it can never be Signed.

The functionality doesn't change. If field_representation is Signed,
it will hit the kNoWriteBarrier at the bottom.

Cq-Include-Trybots: luci.v8.try:v8_linux64_pointer_compression_rel_ng
Cq-Include-Trybots: luci.v8.try:v8_linux64_arm64_pointer_compression_rel_ng
Bug: v8:9396
Change-Id: I39eff3ae082e91d0494134b053c353a2c0ff02c1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1702615Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62890}