intel/compiler: force simd8 when dual src blending on gen8

intel/compiler: force simd8 when dual src blending on gen8

Patch introduces option to force simd8 and uses it as a workaround for
dual source blending issues seen with skqp (skia testsuite) on gen8.

Fixes following Piglit test on gen8 platforms:
   arb_blend_func_extended-dual-src-blending-issue-1917

Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/1917
Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
c: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>