Fix bug in simplify that was skipping saturating arm ops

Merge branch 'master' of https://github.com/halide/Halide

Fixes to old windows build method.

Merge pull request halide#179 from jansel/master

Print warnings to stderr instead of stdout

Merge branch 'master' of github.com:halide/Halide