[dev.power64] 6g: don't create variables for indirect addresses

Previously, mkvar treated, for example, 0(AX) the same as AX.
As a result, a move to an indirect address would be marked as
*setting* the register, rather than just using it, resulting
in unnecessary register moves.  Fix this by not producing
variables for indirect addresses.

LGTM=rsc
R=rsc, dave
CC=golang-codereviews
https://golang.org/cl/164610043

src/cmd/6g/reg.c

@@ -621,6 +621,9 @@ mkvar(Reg *r, Adr *a)
 	if(r != R)
 		r->use1.b[0] |= doregbits(a->index);
 
+	if(t >= D_INDIR && t < 2*D_INDIR)
+		goto none;
+
 	switch(t) {
 	default:
 		regu = doregbits(t);