arch,cpu: Keep a RegClass pointer in RegId instead of a RegClassType.

This makes it easy to get access to the RegClass that goes with a
register without having to look it up in a separate structure.

Change-Id: I4cfff2069d63f3c1c3fb0fea5dee3baf357bd478
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/49786
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/cpu/SConscript

@@ -64,6 +64,7 @@ DebugFlag('ExecAsid', 'Format: Include ASID in trace')
 DebugFlag('ExecFlags', 'Format: Include instruction flags in trace')
 DebugFlag('Fetch')
 DebugFlag('HtmCpu', 'Hardware Transactional Memory (CPU side)')
+DebugFlag('InvalidReg')
 DebugFlag('O3PipeView')
 DebugFlag('PCEvent')
 DebugFlag('Quiesce')