[x86] convert masked store of one element to scalar store

Another opportunity to reduce masked stores: in D16691, we decided not to attempt the 'one mask element is set'
transform in InstCombine, but this should be a win for any AVX machine.

Code comments note that this transform could be extended for other targets / cases.

Differential Revision: http://reviews.llvm.org/D16828

llvm-svn: 260145