regcache: Fix lightning_reg_to_lightrec and lightrec_reg_to_lightning
regcache: Use jit_ldi_i / jit_sti_i instead of jit_ldr_i / jit_str_i

The former are for immediates values (that's what we have here), the
latter are for use with lightning registers.
disassembler: Invert fields of opcode structures

The LSB fields should be written on the top of the structure, not on the
bottom.
disassembler: Set the end of the opcode list to NULL

Otherwise the algorithms iterating on the list won't stop at the end...