Add FIX_1179_USAN_PHASEECU for porting MR1921 into basop

Added FIX_1179_USAN_PHASEECU for porting MR1921 from float. Note that the float code deals with wrap-around, while the BASOP code already saturates. This MR just keeps this behavior, adding an explicit _sat variant for the saturations that happen.