Misc. GPU improvements

• GPU backend fixes.
  • Emit float infinity properly.
  • Emit thread_block_tile variables at start of function.
• RT backend fixes.
  • Lower intrinsics.
  • Emit float infinity properly.
• Add max / min to monoid reduction cleaning (utilities for working w/ smallest/largest values of a datatype).
• Don't outline scalar constants (TODO: inline these interprocedurally later).
• Optimize edge detection schedules, especially GPU.
  • Emit a 2-level max reduction tree.