private Register findRegister(Varnode v, Map<Register, Integer> regIndexMap) {
		if (v.isRegister()) {
			Register reg = getRegister(v.getAddress(), regIndexMap);
			if (reg != null) {
				return reg;
			}
		}
		return null;
	}
Find a register varnode within a map keyed by acceptable register
choices.

@param v varnode
@param regIndexMap register index map
@return register which matches varnode, or null.
Ghidra/Features/Base/src/main/java/ghidra/program/model/listing/CodeUnitFormat.java