Added support for overloading intrinsics (atomics) based on pointers

Added support for overloading intrinsics (atomics) based on pointers
to different address spaces.  This alters the naming scheme for those
intrinsics, e.g., atomic.load.add.i32 => atomic.load.add.i32.p0i32

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@54195 91177308-0d34-0410-b5e6-96231b3b80d8