@Override
			public boolean evaluateContextBefore(VarnodeContext context, Instruction instr) {
				// if the requested reference was on an input op-object, get context before exec
				return checkContext(true, opIndex, context, instr);
			}
Get the register at the location

@param opIndex index into operands for instruction
@param subOpIndex index into operands for an operand location

@return register if there is one at the location
Ghidra/Features/Base/ghidra_scripts/MultiInstructionMemReference.java