ConvertToCFG: use affine_apply to implement loop steps

Originally, loop steps were implemented using `addi` and `constant` operations
because `affine_apply` was not handled in the first implementation.  The
support for `affine_apply` has been added, use it to implement the update of
the loop induction variable.  This is more consistent with the lower and upper
bounds of the loop that are also implemented as `affine_apply`, removes the
dependence of the converted function on the StandardOps dialect and makes it
clear from the CFG function that all operations on the loop induction variable
are purely affine.

PiperOrigin-RevId: 225165337