[ARM] Increase the Scalarized cost of masked gather/scatter operations

If a gather/scatter is masked and will need to be scalarized then the cost
should be higher than we currently produce. An additional cost for scalarizing
the mask, extracting i1s and branching on the result needs to be added, which
this patch gives a cost of 5.

Differential Revision: https://reviews.llvm.org/D147331