[asan-assembly-instrumentation] Prologue and epilogue are moved out from InstrumentMemOperand().

Reviewers: eugenis

Subscribers: llvm-commits

Differential revision: http://reviews.llvm.org/D4923

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216879 91177308-0d34-0410-b5e6-96231b3b80d8