Representability intrinsics not evaluated at compile time

CHERI-MIPS and CHERI-RISC-V both evaluate CRRL/CRAM of constants at compile time (and attempt to do some rudimentary known bits analysis, though I'm not sure if it's currently working), albeit in the backend so you can't use it in a constant expression context (C++ constexpr, or just C global initialisation / array sizes / etc), avoiding unnecessary run-time overhead. This should be done for Morello too.