i965/fs: Properly handle sign(-abs(x))

Fixes new piglit tests:

 - glsl-1.10/execution/fs-sign-neg-abs.shader_test
 - glsl-1.10/execution/fs-sign-sat-neg-abs.shader_test
 - glsl-1.10/execution/vs-sign-neg-abs.shader_test
 - glsl-1.10/execution/vs-sign-sat-neg-abs.shader_test

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
(cherry picked from commit 88bd37c0)