[Target] Use 'unsigned' as the underlying type for the tablegened physical register enums. (#109086)
Otherwise, the enum defaults to 'int'. Update a few places that used
'int' for registers that now need to change to avoid a signed/unsigned
compare warning.
I was hoping this would allow us to remove the 'int' comparison
operators in Register.h and MCRegister.h, but compares with literal 0
still need them.