stage2: fix invalid iteration code in std.ast.Node.Asm

closes #4480

Author: andrewrk

lib/std/zig/ast.zig

@@ -2183,10 +2183,10 @@ pub const Node = struct {
         pub fn iterate(self: *Asm, index: usize) ?*Node {
             var i = index;
 
-            if (i < self.outputs.len) return &self.outputs.at(index).*.base;
+            if (i < self.outputs.len) return &self.outputs.at(i).*.base;
             i -= self.outputs.len;
 
-            if (i < self.inputs.len) return &self.inputs.at(index).*.base;
+            if (i < self.inputs.len) return &self.inputs.at(i).*.base;
             i -= self.inputs.len;
 
             return null;