This CL refactors how the first byte(s) of the input are used to set
internal configuration, like which compiler to use and whether Liftoff
will be used as reference instead of the interpreter.

We now always use exactly one byte, and use it for all internal
configuration. If more bits are needed in the future we can either
extend to two bytes, or use the same bits for multiple things, while
avoiding to lose coverage of all interesting configurations.

For now, we use the first byte to derive
- which compiler to use per function,
- whether to use Liftoff as reference, and
- (new) whether to globally enable the mid-tier register allocator.

R=thibaudm@chromium.org

Bug: v8:12330
Change-Id: I2cae6628554ca8f7e08115015b36f9f0a6b8c34f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3253156
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77929}