[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