This matches the behavior of the HSAIL clock instruction.
s_realmemtime is used if the subtarget supports it, and falls
back to s_memtime if not.
Also introduces new intrinsics for each of s_memtime / s_memrealtime.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@262119 91177308-0d34-0410-b5e6-96231b3b80d8
