When we start tracking the mapping from Value to Prog, valueProgs will
be confusing. Disambiguate.

Change-Id: Ib3b302fedb7eb0ff1bde789d70a11656d82f0897
Reviewed-on: https://go-review.googlesource.com/50876Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>