make memset inference significantly more powerful: it can now handle

memsets that initialize "structs of arrays" and other store sequences
that are not sequential.  This is still only enabled if you pass 
-form-memset-from-stores.  The flag is not heavily tested and I haven't
analyzed the perf regressions when -form-memset-from-stores is passed
either, but this causes no make check regressions.

llvm-svn: 48909