Type after shift in IntelliJ

Maybe this is just a bug in IntelliJ, but let's see if someone has an idea.

**Type of issue**: bug report

Following code
```Scala
val a = 1.U << 1
val b = 3.U
println(a, b)

// IntelliJ editor complains on 'a': Type mismatch, expected UInt, actual: Bits
when(b >= a) {
io.data := 1.U
}
```

displays an error in the IntelliJ editor (a is marked in the when expression). The editor thinks that the shift operation in the first line results in Bits and not a UInt.

However, the code compiles correctly with IntelliJ and also with sbt from the terminal.

What, in the Chisel type system, is confusing the IntelliJ editor here?

**Please tell us about your environment:**
-version 3.1.8
- IntelliJ latest on Mac (just updated)

The example is available here: https://github.com/schoeberl/chisel-playground/blob/master/src/main/scala/issues/Shift.scala