[AArch64][TTI] Improve `LegalVF` when gather loads are scalarized

After determining the cost of loads that could not be coalesced into
`VectorizedLoads` in SLP, computing the cost of a gather-vectorized
load is carried out. Favour a potentially high valid cost when the
type of a group of loads, whose type is a vector of size dependent
upon `VF`, may be legalized into a scalar value.

Fixes: https://github.com/llvm/llvm-project/issues/68953.