Fix substitutions for compare with unfolding

[codyroux]

Right now, the freshen_sub function at

cbat_tools/wp/lib/bap_wp/src/compare.ml

Line 49 in cd3feba

let freshen_sub (rename_map : var Var.Map.t) (sub : Sub.t) : Sub.t =

does not work when invoked with an inline summary of a function call, since the variables there have not been substituted.

The correct solution is to use the existing variable map in Env.t, and freshen those instead.

Alternately, we could save the variable substitution for subroutines in the environment and apply that before analyzing the inlined subroutine, but this is a hack.