Expand IRBuilder interface for atomic memcpy to require pointer alignments. (NFC)

Summary:
 The specification of the @llvm.memcpy.element.unordered.atomic intrinsic requires
that the pointer arguments have alignments of at least the element size. The existing
IRBuilder interface to create a call to this intrinsic does not allow for providing
the alignment of these pointer args. Having an interface that makes it easy to
construct invalid intrinsic calls doesn't seem sensible, so this patch simply
adds the requirement that one provide the argument alignments when using IRBuilder
to create atomic memcpy calls.

llvm-svn: 317918