As now struct and array types are generated arbitrary,
struct.set should only pick mutable fields.

Bug: v8:11954, chromium:1253031
Change-Id: Ic248e93b09ffefd4f5547e102c8ea4d56bb2bd90
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3186439
Commit-Queue: Maria Tîmbur <mtimbur@google.com>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Reviewed-by: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77128}