Scala.js: Remove spurious js.AsInstanceOf around js.LinkTimeIf.

When B <: A and C <: A, `linkTimeIf[A](cond) { B }{ C }` would generate an `.asInstanceOf[A]` that casts the resulting value to a supertype. However, this cast is redundant, as the linker will prune one branch at link time, and the remaining expression is already known to be a compatible type. This commit eliminates the surrounding `.asInstanceOf` around the `linkTimeIf[T]` when both `thenp` and `elsep` are known to be subtypes of `T`. The optimizer already routinely performs this optimization. However, that comes too late for the module instance field alias analysis performed by `IncOptimizer`. In that case, while the desugarer removes the `LinkTimeIf`, the extra `AsInstanceOf` prevents aliasing the field. Removing the cast ahead of time in the compiler allows field aliases to be recognized in the presence of `LinkTimeIf`s. This commit is a forward port of the Scala.js commit scala-js/scala-js@9bb267c

Author: sjrd

compiler/src/dotty/tools/backend/sjs/JSCodeGen.scala

@@ -3321,7 +3321,29 @@ class JSCodeGen()(using genCtx: Context) {
  val genReceiver = genExpr(receiver)
 
  if (sym == defn.Any_asInstanceOf) {
- genAsInstanceOf(genReceiver, to)
+ receiver match {
+ /* This is an optimization for `linkTimeIf(cond)(thenp)(elsep).asInstanceOf[T]`.
+ * If both `thenp` and `elsep` are subtypes of `T`, the `asInstanceOf`
+ * is redundant and can be removed. The optimizer already routinely performs
+ * this optimization. However, that comes too late for the module instance
+ * field alias analysis performed by `IncOptimizer`. In that case, while the
+ * desugarer removes the `LinkTimeIf`, the extra `AsInstanceOf` prevents
+ * aliasing the field. Removing the cast ahead of time in the compiler allows
+ * field aliases to be recognized in the presence of `LinkTimeIf`s.
+ */
+ case Apply(innerFun, List(cond, thenp, elsep))
+ if innerFun.symbol == jsdefn.LinkingInfo_linkTimeIf &&
+ thenp.tpe <:< to && elsep.tpe <:< to =>
+ val genReceiver1 = genReceiver match {
+ case genReceiver: js.LinkTimeIf =>
+ genReceiver
+ case _ =>
+ throw FatalError(s"Unexpected tree $genReceiver is generated for $innerFun at: ${tree.sourcePos}")
+ }
+ js.LinkTimeIf(genReceiver1.cond, genReceiver1.thenp, genReceiver1.elsep)(toIRType(to))(using genReceiver1.pos)
+ case _ =>
+ genAsInstanceOf(genReceiver, to)
+ }
  } else if (sym == defn.Any_isInstanceOf) {
  genIsInstanceOf(genReceiver, to)
  } else {