make memset inference significantly more powerful: it can now handle

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@48909 91177308-0d34-0410-b5e6-96231b3b80d8