Explicitly define bounds of rvar that has been split.

Before this change:

r.min = 0
r.max = 79
for r.outer in [0, 20]
  for r.inner in [0, 4]
    stuff

After this change:
r.min = 0
r.max = 79
for r.outer in [0, 20]
  r.min = r.outer * 4
  r.max = r.outer * 4 + 3
  for r.inner in [0, 4]
    stuff

This affects the bounds of producer stages compute_at r.outer.
Previously they were potentially too large because they thought
r could take on any value from 0 to 79.
Merge branch 'master' of github.com:halide/Halide