Remove extension whitelist from some transforms

Remove extension whitelist from some transforms

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