Currently, IsolateRoot is both the address of the Isolate root and the
base address of the pointer compression reservation. This CL teases the
two uses apart by renaming IsolateRoot to PtrComprCageBase.

- In addition to V8_COMPRESS_POINTERS, add a
  V8_COMPRESS_POINTERS_IN_ISOLATE_CAGE (vs SHARED_CAGE).

- Rename GetIsolate* helpers to GetPtrComprCageBase. When
  V8_COMPRESS_POINTERS_IN_ISOLATE_CAGE is true, the helpers remain as
  aliases to GetPtrComprCageBase.

- Rename kPtrComprIsolateRootAlignment to kPtrComprCageBaseAlignment.

Bug: v8:11460
Change-Id: I1d715f678ce9a0b5731895612ca14f56579b1c48
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2783672
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Auto-Submit: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73790}