public boolean clearBitAt(Instruction instr, Scalar bit, int rightShiftFactor) {
		if (bit != null) {
			int bitNum = (int) bit.getUnsignedValue() - rightShiftFactor;
			return clearBitAt(instr, bitNum);
		}
		value = null;
		return false;
	}
The specified instr has cleared the specified bit for this context reg.
If setting fails the value will be left in an unknown state.
@param instr instruction which has made the bit modification
@param bit the bit to be cleared.
@param rightShiftFactor value will be subtracted from specified bit to determine actual bit
to be cleared. 
@return false if clear not possible (caused by instr not having a fall-through or
this is a multi-bit register without a previous value setting, or bit is null).
Ghidra/Features/Base/src/main/java/ghidra/app/plugin/core/analysis/RegisterContextBuilder.java