Add CommonUniformElim pass

Add CommonUniformElim pass

- UniformElim: Only process reachable blocks

- UniformElim: Don't reuse loads of samplers and images across blocks.
  Added a second phase which only reuses loads within a block for samplers
  and images.

- UniformElim: Upgrade CopyObject skipping in GetPtr

- UniformElim: Add extensions whitelist
  Currently disallowing SPV_KHR_variable_pointers because it doesn't
  handle extended pointer forms.

- UniformElim: Do not process shaders with GroupDecorate

- UniformElim: Bail on shaders with non-32-bit ints.

- UniformElim: Document support for only single index and add TODO.