Remove unnecessary and recursive Space decomposition

Space decomposition recently learnt to decompose prefixes.  Given a
nested definition like in i19031, aggressively trying to decompose while
intersecting can lead to recursive decompositions (building bigger and
bigger nested prefixes).  Turns out the decomposition isn't necessary.

[Cherry-picked 5439e98]

Author: dwijnand

Diff for: compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -209,17 +209,13 @@ object SpaceEngine {
       case (a @ Typ(tp1, _), b @ Typ(tp2, _)) =>
         if isSubType(tp1, tp2) then a
         else if isSubType(tp2, tp1) then b
-        else if canDecompose(a) then intersect(Or(decompose(a)), b)
-        else if canDecompose(b) then intersect(a, Or(decompose(b)))
         else intersectUnrelatedAtomicTypes(tp1, tp2)(a)
       case (a @ Typ(tp1, _), Prod(tp2, fun, ss)) =>
         if isSubType(tp2, tp1) then b
-        else if canDecompose(a) then intersect(Or(decompose(a)), b)
         else if isSubType(tp1, tp2) then a // problematic corner case: inheriting a case class
         else intersectUnrelatedAtomicTypes(tp1, tp2)(b)
       case (Prod(tp1, fun, ss), b @ Typ(tp2, _)) =>
         if isSubType(tp1, tp2) then a
-        else if canDecompose(b) then intersect(a, Or(decompose(b)))
         else if isSubType(tp2, tp1) then a  // problematic corner case: inheriting a case class
         else intersectUnrelatedAtomicTypes(tp1, tp2)(a)
       case (a @ Prod(tp1, fun1, ss1), Prod(tp2, fun2, ss2)) =>

Diff for: tests/pos/i19031.scala

@@ -0,0 +1,9 @@
+//> using options -Werror
+
+sealed trait A:
+  class B extends A
+
+class Test:
+  def t1(a: A): Boolean =
+    a match
+      case b: A#B => true