add truncation operation

It would be nicer to have some kind of subtyping where a
l variable can be used in place of a w variable but it is
more complications than a truncation insutruction.

In some cases, it might be less efficient than the above
solution by requiring two registers instead of one.
(I.e when both the truncated variable and the long variable
live together.)

We have to see what are the usage patterns.

lisc/lisc.h

@@ -130,6 +130,7 @@ enum Op {
 	OAnd,
 	OSext,
 	OZext,
+	OTrunc,
 	OCmp,
 	OCmp1 = OCmp + NCmp-1,
 	OStorel,