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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@123949 91177308-0d34-0410-b5e6-96231b3b80d8