only use index within validlane

Author: vchuravy

src/macros.jl

@@ -327,8 +327,8 @@ function emit_gpu(loop)
             return expr
         end
         loopexpr = quote
-            $(loop.indicies...)
             if __active_lane__
+                $(loop.indicies...)
                 $(unblock(body))
             end
         end