Fix bug introduced by defining asBool

```
[error] .../chisel3/src/main/scala/chisel3/util/ImplicitConversions.scala:16:50: type mismatch;
[error]  found   : x.type (with underlying type Boolean)
[error]  required: ?{def asBool: ?}
[error] Note that implicit conversions are not applicable because they are ambiguous:
[error]  both method fromBooleanToLiteral in package chisel3 of type (x: Boolean)chisel3.package.fromBooleanToLiteral
[error]  and method booleanToBool in object ImplicitConversions of type (x: Boolean)chisel3.Bool
[error]  are possible conversion functions from x.type to ?{def asBool: ?}
[error]   implicit def booleanToBool(x: Boolean): Bool = x.asBool
```