The CPURegister class has an additional "type" field to represent an invalid
register while the cross-platform RegisterBase class uses -1 as a register code.

For consistency and to avoid potential bugs from confusing x0 and
`CPURegister::no_reg()`, use -1 as a register code as well in CPURegister.

Change-Id: I1a36230091433bc854a5966f36e3c25e33a406c1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1813746Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Pierre Langlois <pierre.langlois@arm.com>
Cr-Commit-Position: refs/heads/master@{#63902}