Add eta reduction miniphase

[odersky]

Fixes #10889. Alternative to #13750, from which the test is taken.

[som-snytt]

That was a quickie.

Shorter test with arbitrary constants

import scala.annotation.tailrec
import scala.util.chaining.given

object Test {
  class Ctx
  type Op[A] = Ctx ?=> A

  var min = Int.MaxValue
  var max = 0
  def stk = new Throwable().getStackTrace.length

  @tailrec def f[A](n: Int)(op: Op[A]): A =
    val depth = stk
    min = min.min(depth)
    max = max.max(depth)
    given Ctx = Ctx()
    if (n > 0) f(n-1)(op)
    else op

  def g(ctx: Ctx) = stk

  def main(args: Array[String]) =
    val extra = 3
    f(10)(Ctx ?=> g(summon[Ctx])).tap(res => assert(res <= max + extra, s"min $min, max $max, ran g at $res"))
}

[smarter]

Any fundamental reason this is after erasure but the existing BetaReduce phase is before erasure?

[odersky]

The comment of betaReduce mentions blackbox macros, which would be eliminated after erasure. That might have something to do with it, but I did not dig deeper.

[odersky]

EtaReduce is after erasure since a lot of context function wizardry happens before or at erasure so we want to make sure this is all done before we apply the optimization.

[elfprince13]

This is great, thanks!