Merging r246051:

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r246051 | Matthew.Arsenault | 2015-08-26 14:54:50 -0400 (Wed, 26 Aug 2015) | 6 lines

AMDGPU: Make sure to reserve super registers

I think this could potentially have broken if
one of the super registers were allocated
that contain v254/v255.

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llvm-svn: 253235