m68hc16: Update CPU16 core

- Reduce code duplication by making most loads subroutines in the state machine
- Add ASLW, ASRW, LSRW, ROLW and RORW instructions to emulation
- Fix overflow flag for 16-bit shifts

src/devices/cpu/m68hc16/cpu16.h

@@ -114,6 +114,7 @@ private:
 
 	// misc. state
 	seq m_sequence;
+	seq m_return_sequence;
 	u32 m_ea;
 	u16 m_tmp;
 	bool m_start;