Fixed a bug involving creating branches containing for loops with undefined bodies. Added a test case to test/correctness/specialize_branched_loops that demonstrates the bug.
Minor change to previous commit that causes the branches with undef values to be elided completely.
Added a couple comments to clarify how we handle undefined stmts.
Merge pull request #587 from halide/specialize_branched_loops

Fixed a bug involving creating branches with undefined content.