stop ir-opcode-generated from being dirty in my tree

Summary: At Facebook we don't have the top level .gitignore so this has to live in the hphp dir for us to see its benefit.

Reviewed By: @paulbiss

Differential Revision: D1639452
Fixed crash on debug_backtrace() after a declared parameter was unset

Summary: Repro:
1. Pick a function with at least one parameter.
2. Inside the function, unset the parameter, then eval a code that does `debug_backtrace(..)`.
3. Invoke the function.

Failing test:
  src/Symfony/Bundle/FrameworkBundle/Tests/Templating/Helper/FormHelperTableLayoutTest.php

Reviewed By: @markw65

Differential Revision: D1640030