[ASan] Don't instrument load/stores with !nosanitize metadata.

This is used to avoid instrumentation of instructions added by UBSan
in Clang frontend (see r213291). This fixes PR20085.

Reviewed in http://reviews.llvm.org/D4544.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213292 91177308-0d34-0410-b5e6-96231b3b80d8