Add explicit let statements for the value of the unsplit vars.
Fix bounds computed for split reduction domains.

(bug #464)
Skip adding bounds inference lets at loop levels with no pipelines.

Saves compile time in theory, and makes the IR easier to read.
Add more tests.
Merge branch 'master' into split_rvar_bounds
Merge branch 'master' into split_rvar_bounds
Comment cleanups.
Merge pull request #502 from halide/split_rvar_bounds

Split rvar bounds