Stats minor bug fixes

• loopy/symbolic.py: Removed the indirect-access error as LoopyError and replaced it with ExpressionNotAffineInameError
• loopy/statistics.py:
  • Added support for ignoring a global encountered in LocalMemAccessCounter rather than throwing an error.
  • Added support to catch the ExpressionNotAffineInameError.
  • Some minor fixed to only pay heed to countable insns and ignoring BarrierInstruction
• loopy/diagnostic.py: Added a new Exception ExpressionNotInames