saturated conversion of native GPU data type, larger to narrower

saturated conversion of native GPU data type, larger to narrower

This patch supports saturated conversion of
native GPU data type (char/short/int/float),
from a larger-range data type to a narrower-range data type.
For instance,  convert_uchar_sat(int)

Several test cases are in this patch.

v2: add uint->int, int->uint

Signed-off-by: Homer Hsing <homer.xing@intel.com>
Reviewed-by: "Yang, Rong R" <rong.r.yang@intel.com>