Add better error message in case 'temp var in local?' cannot make a valid decision because of a potential write race.

(Mismatch between Python exclusive and isl inclusive semantics...
Treatment needs to be symmetric for isl.)

Unify two pre-existing code paths for conditional generation.
Compute bounds for 'for' by isl bounds finding.
Use overapproximation *except* when the insn builds its own
  final conditional.
Btw, the axpy nondet failure was a PyOpenCL bug in Array.view.
  (fixed in pyopencl:9207aae)