Useless constraints like LC * 0 == 0 are generated

I found some constraints like 0 * 0 == 0 and LC * 0 == 0 are generated during [my plonkit development](https://github.com/Fluidex/plonkit/issues/3#issuecomment-748452109).

I guess adding a `reduceConstants(ctx);` after [await reduceConstrains(ctx);](https://github.com/iden3/circom/blob/254c4205ac28ca870d7bf64a77ec69e6e0d5b906/src/compiler.js#L103) may be helpful for this problem.