pocl_llvm_api.cc:
- reoder the passes so we can maintain VUA from PHIsToAllocas
- remove some dead code, cleanup some style issues

VariableUniformityAnalysis:
- in case an alloca is written from inside a diverging BB,
  the alloca is varying closes #94
- add preserve analysis for VUA in various passes to avoid losing
  the info that gets lost in PHIsToAllocas

PHIsToAllocas:
- maintain information of the split PHIs in VUA to not lose
  track of the loop induction variables after the PHI breaking