cc: restore side(), but do not consider OINDEX as side effect free

from charles forsuth:

because the previous version thought OINDEX might have a side effect, it
stopped it building a tower of them.
probably the best thing is to limit that anyway, since each one consumes
2-3 registers, so towering them can
keep even more active, and the x86 hasn't got that many.

the quick hack is to return that case to the earlier state by treating
OINDEX as a side-effect in side().
it's not a bad thing to do in the OSTRUCT case, for similar reasons: it's
better to collapse the indexed pointer
into a direct register, instead of repeating the indexing operation through
the copying of the value.
OINDEX isn't a machine-independent operation, so it doesn't affect the uses
in ../cc

sys/src/cmd/cc/sub.c

@@ -961,6 +961,10 @@ loop:
 	case OSTRING:
 	case OLSTRING:
 	case ONAME:
+	case OREGPAIR:
+	case OEXREG:
+	case OREGISTER:
+	case OINDREG:
 		return 0;
 	}
 	return 1;