[AArch64][CostModel] Make sext/zext free if folded into a masked load

The BasicTTIImpl implementation of getCastInstrCost ensures
that the cost of zext/sext is 0 when following a load if we
know the combined extending load is legal. For SVE we can do
the same for masked loads too, since they use exactly the
same underlying instruction.

Differential Revision: https://reviews.llvm.org/D148123