Extend Phi placement to support unstructured control flow

This is a pre-requisite for #889. There exists an implementation for Phi placement in https://github.com/KhronosGroup/SPIRV-Tools/blob/master/source/opt/local_ssa_elim_pass.cpp#L410. However, that implementation is limited to structured control flow input.

To compute the minimal or pruned SSA form needed to insert Phi instructions, implement the algorithms referenced in https://en.wikipedia.org/wiki/Static_single_assignment_form.

This will require

- [ ] Implementation of a dominator tree.
- [ ] Implementation of dominance frontiers.