[clang/asan] Do not emit memcpy for trivial operator= when -fsanitize-address-field-padding >= 1

Summary: If we've added poisoned paddings to a type do not emit memcpy for operator=.

Test Plan: regression tests.

Reviewers: majnemer, rsmith

Reviewed By: rsmith

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D6160

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@221739 91177308-0d34-0410-b5e6-96231b3b80d8