Add pass number to dot dump filename

Summary:
Change .dot dumps filename format from
  <function>-<passname>.dot
to
  <function>-<passidx>_<passname>.dot
This change helps navigate dumps by making the pass order explicit.
Example:
  execute_stack_op.cold.6-1(*2)-00_build-cfg.dot
  execute_stack_op.cold.6-1(*2)-01_validate-internal-calls.dot
  execute_stack_op.cold.6-1(*2)-02_strip-rep-ret.dot
  ...

(cherry picked from FBD24452903)