Optimize BytecodeArrayRandomIterator to reserve roughly the right
size index array based on bytecode array length. Also save the
bytecode length in BytecodeArrayAccessor to avoid a more expensive
heap read accessor on BytecodeArray.

BUG=v8:9684

Change-Id: I7f85439877dbfc5ccf5aacc9d4006bd285f1c891
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2593330
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Auto-Submit: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71772}