Merging r203054:

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r203054 | hfinkel | 2014-03-05 20:28:23 -0500 (Wed, 05 Mar 2014) | 7 lines

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.

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llvm-svn: 205823