Backport "Fix #20521: Optimise caching for computing atoms and widened in OrTypes" to LTS (#22073)

Backports #21223 to the 3.3.5.

PR submitted by the release tooling.
[skip ci]

Author: WojciechMazur

Diff for: compiler/src/dotty/tools/dotc/core/Types.scala

@@ -3510,8 +3510,9 @@ object Types extends TypeUtils {
       myUnion
 
     private var atomsRunId: RunId = NoRunId
-    private var myAtoms: Atoms = _
-    private var myWidened: Type = _
+    private var widenedRunId: RunId = NoRunId
+    private var myAtoms: Atoms = uninitialized
+    private var myWidened: Type = uninitialized
 
     private def computeAtoms()(using Context): Atoms =
       if tp1.hasClassSymbol(defn.NothingClass) then tp2.atoms
@@ -3523,18 +3524,16 @@ object Types extends TypeUtils {
       val tp2w = tp2.widenSingletons
       if ((tp1 eq tp1w) && (tp2 eq tp2w)) this else TypeComparer.lub(tp1w, tp2w, isSoft = isSoft)
 
-    private def ensureAtomsComputed()(using Context): Unit =
+    override def atoms(using Context): Atoms =
       if atomsRunId != ctx.runId then
         myAtoms = computeAtoms()
-        myWidened = computeWidenSingletons()
         if !isProvisional then atomsRunId = ctx.runId
-
-    override def atoms(using Context): Atoms =
-      ensureAtomsComputed()
       myAtoms
 
     override def widenSingletons(using Context): Type =
-      ensureAtomsComputed()
+      if widenedRunId != ctx.runId then
+        myWidened = computeWidenSingletons()
+        if !isProvisional then widenedRunId = ctx.runId
       myWidened
 
     def derivedOrType(tp1: Type, tp2: Type, soft: Boolean = isSoft)(using Context): Type =