This CL moves the files in opt/ to consistenly be under the opt::
namespace. This frees up the ir:: namespace so it can be used to make a
shared ir represenation.
Remove extension whitelists from transforms that are essentially
combinatorial (and avoiding pointers) or which affect only control flow.
It's very very unlikely an extension will add a new control flow construct.
Remove from:
- dead branch elimination
- dead insertion elimination
- insert extract elimination
- block merge
Fixes https://github.com/KhronosGroup/SPIRV-Tools/issues/1392
