regcache: Add function lightrec_storeback_all_regs

This function will invalidate all the registers. The behaviour of the
previous function lightrec_storeback_regs has changed, and will now
invalidate only caller-saved registers.
regcache: Use caller-saved registers in priority for temporaries
regcache: Use const pointers when possible
recompiler: Renamed function rec_opcode to lightrec_rec_opcode
regcache: Allow one register to be reused