[X86] Change the addcarry and subborrow intrinsics to return 2 results and...

[X86] Change the addcarry and subborrow intrinsics to return 2 results and remove the pointer argument.

We should represent the store directly in IR instead. This gives the middle end a chance to remove it if it can see a load from the same address.

Differential Revision: https://reviews.llvm.org/D51769

llvm-svn: 341677