regcache: Replace lightrec_free_reg by lightrec_free_regs

This new function will free all allocated registers.
regcache: Add some jit_note for debugging purposes
regcache: Added a lightrec_rvals array containing known values of regs
regcache: Added function lightrec_alloc_reg_temp_with_value

This function can be used to request a temporary register containing a
specific value.
regcache: Optimize loading/storing in register cache

A temporary register is now used to memorize the upper 16-bit of the
address of the register_cache array. Then, LDXI/STXI can be used to load
from or store to the contained address, using the lower 16-bit as
offset.
regcache: Avoid using a register that may be used later
emitter: Added example recompiling function rec_special_ADDU