cmd/7g: fix x := a / (-1)

Fixes golang#20
Fixes golang#19

Also includes a fix for the compiler segfault. Minux says that MOV R0, R0 might get eaten by the peep opt, which is probably true, but I'm not sure if the defer magic happens before peep or not.

Author: davecheney

src/cmd/7g/ggen.c

@@ -198,8 +198,8 @@ ginscall(Node *f, int proc)
 				// ARM64 NOP is really HINT $0
 				// Use the latter form because the NOP pseudo-instruction
 				// would be removed by the linker.
-				nodconst(&con, types[TINT], argsize(f->type));
-				gins(AHINT, &con, N);
+				nodreg(&reg, types[TINT], D_R0);
+				gins(AMOV, &reg, &reg);
 			}
 			p = gins(ABL, N, f);
 			afunclit(&p->to, f);
@@ -571,7 +571,8 @@ dodiv(int op, Node *nl, Node *nr, Node *res)
 		p1 = gbranch(optoas(ONE, t), T, +1);
 		if(op == ODIV) {
 			// a / (-1) is -a.
-			gins(optoas(OMINUS, t), N, &tl);
+			nodconst(&nz, t, 0);
+			gins(optoas(OMINUS, t), &nz, &tl);
 			gmove(&tl, res);
 		} else {
 			// a % (-1) is 0.