cpu: constexpr most of RegId's methods, and add a RegIndex operator.

That will let a RegId be used where a RegId is required, but also let it
be downconverted into a scalar RegIndex if using an older API. Note that
this does *not* let you automatically upconvert from a RegIndex into a
RegId, since there would be no way to know what class of register to

Change-Id: I5fff224dce5e02959d5fc3e717014bf7eaa9c022
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/49753
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
1 file changed