[X86] Modify the the rdtscp intrinsic to return values instead of taking a pointer argument

Similar to what was recently done for addcarry/subborrow and has been done for rdrand/rdseed for a while. It's better to use two results and an explicit store in IR when the store isn't part of the semantics of the instruction. This allows store->load forwarding to happen in the middle end. Or the store to be removed if its never loaded.

Differential Revision: https://reviews.llvm.org/D51803

llvm-svn: 341698