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)

Thanks, c0d784d3.

- This (plus a few more fixes) restore race detection.

- Also get rid of sequential_inames--realize that by a new tag,
  force-sequential.

- Also move loop_nest_map to the scheduler file, because it tweaks
  its results to suit scheduler internals.

Conflicts:
	loopy/kernel.py