Remove dead lets also for very simple cases like "let var = constant".

Without removing such lets, later calls to unify_duplicate_lets() can
incorrectly reintroduce variables for constants: For example,

let var = 0
  0

will be turned into

let var = 0
  var
Fix typos in comments
Merge pull request #224 from halide/daniel-bugfixes

Fix one bug in Simplify and two small typos