tcg/TODO

- Add new instructions such as: setcond, clz, ctz, popcnt.
- See if it is worth exporting mul2, mulu2, div2, divu2. 
- Support of globals saved in fixed registers between TBs.
Ideas:
- Move the slow part of the qemu_ld/st ops after the end of the TB.
- Change exception syntax to get closer to QOP system (exception
  parameters given with a specific instruction).
- Add float and vector support.