model: bugfix - inherit future values from previous loads

model: bugfix - inherit future values from previous loads

Once a future value is observed once, it should be available for
observation by other loads. Previously, we assume that a future value
will be passed to each load that may observe it. However, as
exemplified in the double-read-fv.c test, this may not be possible when
there are no other feasible stores from which to read.

Thus, when building an initial 'may-read-from' set, we should build a
set of inherited future values as well.