Simplify operands of masked stores and scatters based on demanded elements

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@356590 91177308-0d34-0410-b5e6-96231b3b80d8