CheckInlineConformant is too strict for -Ycheck:frontend

[gsps]

object BadInlineConstCheck {
  final val MaxSize = Int.MaxValue + 0
}

crashes in checkInlineConformant:

Exception in thread "main" class dotty.tools.dotc.reporting.diagnostic.messages$Error at try/BadInlineConstCheck.scala:2: right-hand side of inline value MaxSize must be a constant expression or a function
	at dotty.tools.dotc.reporting.Reporting.error(Reporter.scala:88)
	at dotty.tools.dotc.reporting.Reporting.error$(Reporter.scala:87)
	at dotty.tools.dotc.core.Contexts$Context.error(Contexts.scala:70)
	at dotty.tools.dotc.typer.Checking.checkInlineConformant(Checking.scala:607)
	at dotty.tools.dotc.typer.Checking.checkInlineConformant$(Checking.scala:600)
	at dotty.tools.dotc.typer.Typer.checkInlineConformant(Typer.scala:82)
	at dotty.tools.dotc.typer.Typer.$anonfun$typedValDef$1(Typer.scala:1352)
(...)

The problem here lies in the isPureExpr check returning false, which seems to be specifically related to the select of Int.MaxValue not being considered pure. Ultimately, refPurity of a reference to the Int module return Idempotent, which makes the overall expression idempotent rather than pure.

The standard settings of -Ycheck don't run into this crash, since they (I think) only check after some simplification on the trees has taken place, replacing the selection by a simple literal of the same constant value.

[smarter]

Note that this only happens on master when doing -Ycheck:frontend, #725 tracks the remaining test cases that need to be fixed before we turn it on, a few of them are affected by this issue.