[SLP] Don't vectorize stores of non-packed types (like i1, i2)

In the spirit of commit fc783e91 (llvm-svn: 248943) we
shouldn't vectorize stores of non-packed types (i.e. types that
has padding between consecutive variables in a scalar layout,
but being packed in a vector layout).

The problem was detected as a miscompile in a downstream test case.

Reviewed By: anton-afanasyev

Differential Revision: https://reviews.llvm.org/D94446