Also add a V8_COMPRESS_POINTERS_IN_SHARED_CAGE define when pointer
compression is enabled.

This CL is to get performance numbers for reserving an extra register.
There is no actual pointer cage yet, and the base register will always
have the same value as the root register. The pointer decompression code
is switched to using the base register instead of the root register.

Bug: v8:11460
Change-Id: I40bae556c2098608fb6fc193a52694e3f54754bd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2716075Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73204}