GH-116202: Incorporate invalidation check into _START_EXECUTOR.

[markshannon]

The uop pair stats show that _START_EXECUTOR _CHECK_VALIDITY is the second most common pair and that every _START_EXECUTOR is followed by either a _CHECK_VALIDITY or _CHECK_VALIDITY_SET_IP.

This PR adds a validity check to _START_EXECUTOR allowing the optimizer to remove the redundant _CHECK_VALIDITYs.
This also reduces the number of validity checks in loops as traces are known to be valid at _JUMP_TO_TOP.

  _START_EXECUTOR
top:
  _CHECK_VALIDITY
  ...
  _JUMP_TO_TOP

becomes:

  _START_EXECUTOR
top:
  ...
  _JUMP_TO_TOP

• Issue: Tier 2 superuops #116202

[gvanrossum]

Makes sense!