Handle invalid cancellation signals in trymerge (#110690)
This change is needed after https://github.com/pytorch/test-infra/pull/4579 and https://github.com/pytorch/test-infra/pull/4610. All invalid cancelled signals have been removed from Dr.CI and HUD. So trymerge should ignore them accordingly for a consistent experience.
### Testing
https://github.com/pytorch/pytorch/pull/110367#issuecomment-1750099960 is the PR where a bunch of invalid cancelled signals showed up and blocked merges
Pull Request resolved: https://github.com/pytorch/pytorch/pull/110690
Approved by: https://github.com/clee2000, https://github.com/ZainRizvi
