mep: ubsan: mep-ibld.c:1635,1645,1652 left shift of negative value

cpu/
	* mep-core.cpu (f-8s8a2, f-12s4a2, f-17s16a2): Multiply signed
	value by two rather than shifting left.
	(f-24s5a2n): Similarly multiply signed f-24s5a2n-hi to extract.
opcodes/
	* mep-ibld.c: Regenerate.