[Verifier] Remove invalid verifier check

According to LangRef for unordered atomic memory transfer intrinsics
"The first three arguments are the same as they are in the @llvm.memcpy intrinsic, with the added constraint that
 len is required to be a positive integer multiple of the element_size. If len is not a positive integer multiple
 of element_size, then the behaviour of the intrinsic is undefined."

So the len is not multiple of element size is just an undefined behavior and verifier should not complain about that
as undefined behavior is allowed in LLVM IR.

This change removes the verifier check for this condition

Reviewers: reames
Reviewed By: reames
Subscribers: dantrushin, hiraditya, jfb, llvm-commits
Differential Revision: https://reviews.llvm.org/D76116