[LV] Use cost base decision for uniform mem op strategy [nfc-ish]

This is mostly a stylistic change to make the uniform memop widening cost
code fit more naturally with the sourounding code.  Its not strictly
speaking NFC as I added in the store with invariant value case, and we
could in theory have a target where a gather/scatter is cheaper than a
single load/store... but it's probably NFC in practice.  Note that the
scatter/gather result can still be overriden later if the result is
uniform-by-parts.