[CostModel][AArch64] Make loads/stores of <vscale x 1 x eltty> invalid.

At the moment, <vscale x 1 x eltty> are not yet fully handled by the
code-generator, so to avoid vectorizing loops with that VF, we mark the
cost for these types as invalid.
The reason for not adding a new "TTI::getMinimumScalableVF" is because
the type is supposed to be a type that can be legalized. It partially is,
although the support for these types need some more work.

Reviewed By: paulwalker-arm, dmgreen

Differential Revision: https://reviews.llvm.org/D103882