Use the PPC no-r0 class on the TOC LD pseudos

The register parameter in these instructions becomes the base register in an
r+i ld instruction (and, thus, cannot be r0).

This is not yet testable because we don't yet allocate r0 (and even then any
test would be very fragile).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178121 91177308-0d34-0410-b5e6-96231b3b80d8