Refactor OptimizeIndexing (#100549)
This PR decouples the logic necessary to compute bounds on variables
from the logic that uses this info to perform the strenght analysis on
int64 variables. While doing so, it tries to minimize the number of
attributes of the class in favour of local variables.
This class is now accessible from any `LoopBody` object.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/100549
Approved by: https://github.com/eellison
