Make a unified code generation strategies to take place of FIFOPush/Pop and Trigger counter.

[were]

• A unified trigger counter generation.
  • Trigger counter can be written in a downstream module.
  • Write a compiler pass to rewrite wait-until with a trigger counter.
  • Generate downstream and module with different triggering condition
  • Have both simulator and verilator backend tested.
• FIFO unified generation.
  • Similar with above.