Added error when forcing SIMD width of 8 on platforms that support at
least 16. Earlier using IGC_ForceOCLSIMDWidth=8 flag on PVC caused
segfault instead of communicating to user that forcing SIMD width of 8
is not supported on current platform. Issue was addressed in earlier PR
but error wasn't properly implemented. This PR fixes this error. Ocloc
test for error funcionality was added by Kacper Guzewicz
(kacper.guzewicz@intel.com).

(cherry picked from commit 900db4e123ceacc21d052c2f1f29a6ef698d0cc4)