ac/nir: add a workaround for bitfield_extract when count is 0

ac/nir: add a workaround for bitfield_extract when count is 0

LLVM 7 returns incorrect results when count is 0, something
has been broken since LLVM 6. Of course, the best solution is
to fix LLVM but this workaround works as expected for now.

Original workaround by Philippe Rebohle.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107276
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>