[TTI] Change TargetTransformInfo::getMinimumVF to return ElementCount

This will be needed in the loop-vectorizer where the minimum VF
requested may be a scalable VF. getMinimumVF now takes an additional
operand 'IsScalableVF' that indicates whether a scalable VF is required.

Reviewed By: kparzysz, rampitec

Differential Revision: https://reviews.llvm.org/D96020