Implement VirtualRegister::EffectiveWriteSize and VirtualRegister::PreservesBytesOnWrite for flags and stack pointer registers

[pgoodman]

Both these registers are currently considered to be KIND_ARCH_FIXED, and therefore are not considered scalable. But this isn't consistent with reality, and as more things are tracked by the SSA tracker, this could turn into a bug! Therefore, this must be dealt with.