[LoopVectorize] Fix cost for calls to functions that have vector versions

A recent commit
(https://reviews.llvm.org/rG66c120f02560ef528a60924104ead66f330190f1) changed
the cost for calls to functions that have a vector version for some
vectorization factor. However, no check is performed for whether the
vectorization factor matches the current one being cost modeled. This leads to
attempts to widen call instructions to a vectorization factor for which such a
function does not exist, which in turn leads to an assertion failure.

This patch adds the check for vectorization factor (i.e. not just that the
called function has a vector version for some VF, but that it has a vector
version for this VF).

Differential revision: https://reviews.llvm.org/D74944