[LangRef] Memset/memcpy/memmove can take undef/poison pointer if the size is 0

According to the current LangRef, Memset/memcpy/memmove can take a
null/dangling pointer if the size is zero.
(Relevant thread: http://lists.llvm.org/pipermail/llvm-dev/2017-July/115665.html )
This patch expands it and allows the functions to take undef/poison pointers
too.

This required the updates in the align attribute since it isn't specified
what is the alignment of undef/poison pointers.
This patch states that their alignment is 1.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D86643