Don't be overly aggressive with CSE of "ldr constantpool". If it's a pc-relative

value, the "add pc" must be CSE'ed at the same time. We could follow the same
approach as T2 by adding pseudo instructions that combine the ldr + "add pc".
But the better approach is to use movw + movt (which I will enable soon), so
I'll leave this as a TODO.

llvm-svn: 123949