CodeGen: Loads/stores to allocas for atomic ops shouldn't be volatile

Don't inherit the volatile-ness of the input pointer to the volatile
operation for memory allocated on the side.

This fixes PR17306.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224110 91177308-0d34-0410-b5e6-96231b3b80d8