[LV] Make sure the MaxVF is a power-of-2 by rounding down.

LV currently only supports power of 2 vectorization factors, which has
been made explicit with the assertion added in
84045054.

However, if the widest type is not a power-of-2 the computed MaxVF won't
be a power-of-2 either. This patch updates computeFeasibleMaxVF to
ensure the returned value is a power-of-2 by rounding down to the
nearest power-of-2.

Fixes PR46139.

Reviewers: Ayal, gilr, rengolin

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D80870