boolean setValueAt(Instruction instr, Address valueFrom, boolean overwrite) {
		if (value == null || overwrite) {
			BigInteger val = program.getProgramContext().getValue(reg, valueFrom, false);
			if (val != null) {
				setValueAt(instr, val, true);
				return true;
			}
			setValueUnknown();
		}
		return false;
	}
Set an assumed register value at the specified instr using
the register value at valueFrom.  If overwrite is true and a 
context value is not found at valueFrom, the value state will revert 
to unknown.
@param instr first instruction of range
@param valueFrom point from which existing register value should
be read from program context.
@param overwrite this method will take no action and return false if 
this parameter is false and a context value has previously been set,
otherwise an attempt will be made to overwrite the current value state.
@return true if value was set, otherwise false.
Ghidra/Features/Base/src/main/java/ghidra/app/plugin/core/analysis/RegisterContextBuilder.java