Account for `@showAsInfix` in `isInfixType`

[This TODO](https://github.com/dotty-staging/dotty/blob/7bfe4d5241c4e4c1f15e18ea950b23f738f4c683/compiler/src/dotty/tools/dotc/printing/RefinedPrinter.scala#L132)

https://github.com/scala/scala/blob/v2.12.3/src/library/scala/annotation/showAsInfix.scala