Warning occurs when the decoder fields chiselType is chiselEnum

warning is generated when the decoder fields chiselType is chiselEnum
The warning for example is:
```
[119] [warn] src/main/scala/chisel3/util/experimental/decode/DecoderBundle.scala 88:106: [W001] Casting non-literal UInt to untitl3d.backend.ImmTpe. You can use untitl3d.backend.ImmTpe.safe to cast without this warning.
```
the code in `src/main/scala/chisel3/util/experimental/decode/DecoderBundle.scala 88:106` is:
``` scala
def decode(input: UInt): DecodeBundle = chisel3.util.experimental.decode.decoder(input, table).asTypeOf(bundle)
```
the warning occurs in the `asTypeOf(bundle)`, because there has chiselEnum in bundle
(but it just a small problem i think

Reproduce the warning from src/main/scala/chisel3/util/experimental/decode/DecoderBundle.scala, focusing on the decode method and its asTypeOf(bundle) call when the bundle contains a chiselEnum. Trace why this cast emits the warning, then verify that decoding such a bundle no longer produces the reported warning.

Written by the indexing model from the issue text.