Improvements to the implicit vectorization

- do not unroll loops in the clBuildProgram stage to present nicer
  loops to the vectorizer (it can decide what to do with them)
- note: loopvec is the default WG method now
- print out vectorization diagnostics
- add an XFAIL test case for an issue that happens only when vectorization is
  enabled:
  sub_sat 3-element vector cases fail when vectorizer is enabled,
  at least with Intel Core i5.
- tce: fp16 test fails with loopvec, seems like a codegen issue. XFAIL for now.
Re-entrance and a comment fix
POCL_VECTORIZER_REMARKS option

POCL_VECTORIZER_REMARKS:
When set to 1, prints out remarks produced by the loop vectorizer of
LLVM during kernel compilation.

Clean up the alphabetical order of the env variable descriptions and
other minor doc updates.
Add the WG method to the kernel compiler hash key