cmd/8c: better fix for 64-bit register smash

Ken pointed out that CL 5998043 was ugly code.
This should be better.

Fixes #3501.

R=ken2
CC=golang-dev
https://golang.org/cl/6258049

src/cmd/8c/cgen.c

@@ -1703,6 +1703,7 @@ copy:
 		}
 	}
 
+	v = w == 8;
 	if(n->complex >= FNX && nn != nil && nn->complex >= FNX) {
 		t = nn->type;
 		nn->type = types[TLONG];
@@ -1728,8 +1729,28 @@ copy:
 	}
 
 	x = 0;
-	v = w == 8;
 	if(v) {
+		if(nn != nil && nn->complex >= FNX) {
+			t = nn->type;
+			nn->type = types[TLONG];
+			regialloc(&nod2, nn, Z);
+			lcgen(nn, &nod2);
+			nn->type = t;
+			
+			nod2.type = typ(TIND, t);
+	
+			nod1 = nod2;
+			nod1.op = OIND;
+			nod1.left = &nod2;
+			nod1.right = Z;
+			nod1.complex = 1;
+			nod1.type = t;
+	
+			sugen(n, &nod1, w);
+			regfree(&nod2);
+			return;
+		}
+			
 		c = cursafe;
 		if(n->left != Z && n->left->complex >= FNX
 		&& n->right != Z && n->right->complex >= FNX) {

src/cmd/8c/cgen64.c

@@ -1601,33 +1601,6 @@ cgen64(Node *n, Node *nn)
 		prtree(n, "cgen64");
 		print("AX = %d\n", reg[D_AX]);
 	}
-
-	if(nn != Z && nn->complex >= FNX) {
-		// Evaluate nn address to register
-		// before we use registers for n.
-		// Otherwise the call during computation of nn
-		// will smash the registers.  See
-		// http://golang.org/issue/3501.
-
-		// If both n and nn want calls, refuse to compile.
-		if(n != Z && n->complex >= FNX)
-			diag(n, "cgen64 miscompile");
-
-		reglcgen(&nod1, nn, Z);
-		m = cgen64(n, &nod1);
-		regfree(&nod1);
-		
-		if(m == 0) {
-			// Now what?  We computed &nn, which involved a
-			// function call, and didn't use it.  The caller will recompute nn,
-			// calling the function a second time.
-			// We can figure out what to do later, if this actually happens.
-			diag(n, "cgen64 miscompile");
-		}
-
-		return m;
-	}
-
 	cmp = 0;
 	sh = 0;