[asan] properly instrument memory accesses that have small alignment (smaller than min(8,size)) by making two checks instead of one. This may slowdown some cases, e.g. long long on 32-bit or wide loads produced after loop unrolling. The benefit is higher sencitivity.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209508 91177308-0d34-0410-b5e6-96231b3b80d8