`UseScratchRegisterScope` should be used in a block scope, to avoid keeping the
scratch register in use for longer than it's needed. Spotted when experimenting
with the `v8_enable_heap_sandbox` and `v8_enable_external_code_space` flags.

Change-Id: I34330f3e4dbd114aa75efb0ebdcf557f0bd45f11
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3256997Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Georgia Kouveli <georgia.kouveli@arm.com>
Cr-Commit-Position: refs/heads/main@{#77679}