The PPC global base register cannot be r0
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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@203054 91177308-0d34-0410-b5e6-96231b3b80d8

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