The PPC global base register cannot be r0

The global base register cannot be r0 because it might end up as the first
argument to addi or addis. Fixes PR18316.

I don't have a small stable test case.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203054 91177308-0d34-0410-b5e6-96231b3b80d8