radv/ac: workaround regression in llvm 4.0 release

radv/ac: workaround regression in llvm 4.0 release

LLVM 4.0 released with a pretty messy regression, that hopefully
get fixed in the future.

This work around was proposed by Tom, and it fixes the CTS regressions
here at least, I'm not sure if this will cause any major side effects,
but correctness over speed and all that.

radeonsi should possibly consider the same workaround until an llvm
fix can be found.

Acked-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Signed-off-by: Dave Airlie <airlied@redhat.com>