[Feature] Register assignment/allocation: need a real implementation

[markoshorro]

Description of the problem or scenario potentially improvable:
Currently the implementation of the register allocation is inexistent: there is only a set of maps/sets keeping track of the variables within the VectorIR. This is not good implementation.

Description of the solution

There must be, at least, one pass or stage of the algorithm keeping track of this.

Side effects or any other known issues caused by this feature:

The number of variables could be highly optimized, generating more compact and readable code.