Implement better register allocation

[hpcharles]

The current implementation use

• tmp register for variable allocation then
• allocate dynamically, possibly with callee saved register -> since the allocation is dynamic, the callee save register set is dynamic

Idea:

• Implement variable allocation in callee saved register -> the number is known at static compile time & allow to use stack handling
• Then :
  • Use tmp register, without stack saved
  • Use saved register & save it at 1st use, & mark it as "should be restored"