Fix store collector for upsample pattern

The store collector was bailing out when it found too many stores,
rather than only considering the first few. e.g:

f[ramp(a, 2, 4)] = ...
f[ramp(a+1, 2, 4)] = ...
f[ramp(b, 2, 4)] = ...
f[ramp(b+1, 2, 4)] = ...

can be rewritten into two interleaving stores, but the store collector
was finding four stores and bailing entirely.