I don't think pH can affect AgCl significantly because it has a strong acid's conjugate, Cl-.
The dissolution equation of AgCl would be AgCl <--> Ag+ + Cl-
If you added H+, the solubility wouldn't be affected because HCl is a strong acid (100% ionization).
If you had something like AgF...
AgF <--> Ag+ + F-
If you added H+, the solubility will increase. H+ will remove F- from solution to form HF (weak acid with very low % ionization), shifting equilibrium to the right and favoring dissolution.
AgCl dissolves in HCl because it forms a complex ion, like the above poster said.
However, AgCl doesn't dissolve in a strong acid like HNO3