[LV] Introduce TTI::getMinimumVF

The function getMinimumVF(ElemWidth) will return the minimum VF for
a vector with elements of size ElemWidth bits. This value will only
apply to targets for which TTI::shouldMaximizeVectorBandwidth returns
true. The value of 0 indicates that there is no minimum VF.

Differential Revision: https://reviews.llvm.org/D45271

llvm-svn: 330062