[VPlan] Relax IV user assertion after 0ab539fd for epilogue vec.

After 0ab539fd, the canonical IV in the
epilogue vector loop may be used by a trunc. Relax the corresponding
assert.

This should fix some build-bot failures, including
    https://lab.llvm.org/buildbot/#/builders/187/builds/14113
    https://lab.llvm.org/buildbot/#/builders/98/builds/32350
    https://lab.llvm.org/buildbot/#/builders/239/builds/5473