Simplify operands of masked stores and scatters based on demanded elements

If we know we're not storing a lane, we don't need to compute the lane. This could be improved by using the undef element result to further prune the mask, but I want to separate that into its own change since it's relatively likely to expose other problems.

Differential Revision: https://reviews.llvm.org/D57247

llvm-svn: 356590