cpu: Treat the InvalidRegClass like the zero register.

This is a transitional step towards the InvalidRegClass taking over for
the zero register.

Change-Id: I423e1f6b5138d8bb41493f9febb3b28f333f9f00
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/49744
Maintainer: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>
14 files changed