Add element atomic memmove intrinsic

Summary: Continuing the work from https://reviews.llvm.org/D33240, this change introduces an element unordered-atomic memmove intrinsic. This intrinsic is essentially memmove with the implementation requirement that all loads/stores used for the copy are done with unordered-atomic loads/stores of a given element size.

Reviewers: eli.friedman, reames, mkazantsev, skatkov

Reviewed By: reames

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34884

llvm-svn: 307796