Fix for regression: [Bug 20369] wrong code at -O3 on x86_64-linux-gnu in 64-bit mode

Prevents hoisting of loads above stores and sinking of stores below loads
in MergedLoadStoreMotion.cpp (rdar://15991737)



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213497 91177308-0d34-0410-b5e6-96231b3b80d8