Avoid erasure/preErasure issues around Any in transformIsInstanceOf

The testType Any is erased to Object, but the expr type Int isn't erased
to Integer, so then it fails as Int !<: Object.  We avoid the problem by
feeding in AnyVal, leading to a (possibly elided) non-null test only.

[Cherry-picked d02d4b8]

Author: dwijnand

compiler/src/dotty/tools/dotc/transform/TypeTestsCasts.scala

@@ -352,7 +352,11 @@ object TypeTestsCasts {
                 report.error(em"$untestable cannot be used in runtime type tests", tree.srcPos)
                 constant(expr, Literal(Constant(false)))
               case _ =>
-                val erasedTestType = erasure(testType)
+                val erasedTestType =
+                  if testType.isAny && expr.tpe.isPrimitiveValueType then
+                    defn.AnyValType
+                  else
+                    erasure(testType)
                 transformIsInstanceOf(expr, erasedTestType, erasedTestType, flagUnrelated)
         }
 

tests/pos/i21544.scala

@@ -0,0 +1,2 @@
+class Test():
+  def m1(xs: List[Boolean]) = for (x: Any) <- xs yield x