[IR] Relax convergence requirements on call (#135794)
Before this commit, having a convergence token on a non-convergent call
was considered to be an error.
This commit relaxes this requirement and allows convergence tokens to be
present on non-convergent calls.
When such token is present, they have no effect as the underlying call
is non-convergent.
This allows passes like DCE to strip `convergent` attribute from
functions for which all convergent operations have been stripped. When
this happens, a convergence token can still exist in the call-site,
causing the verifier to complain.
Alternatives have been considered in #134863 and #134844.
