Fixing @llvm.memcpy not honoring volatile.

This is explicitly not addressing target-specific code, or calls to memcpy.

Summary: https://bugs.llvm.org/show_bug.cgi?id=42254

Reviewers: courbet

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63215

llvm-svn: 365449