[LV] Use umax(VF * UF, MinProfTC) for scalable vectors.

For scalable vectors, it is not sufficient to only check
MinProfitableTripCount if it is >= VF.getKnownMinValue() * UF, because
this property may not holder for larger values of vscale. In those
cases, compute umax(VF * UF, MinProfTC) instead.

This should fix
https://lab.llvm.org/buildbot/#/builders/197/builds/2262