Use nextUnsharedTag to simplify the code

Author: liufengyun

compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala

@@ -1062,13 +1062,7 @@ class TreeUnpickler(reader: TastyReader, tastyName: TastyName.Table, posUnpickle
       }
 
     def readCases(end: Addr)(implicit ctx: Context): List[CaseDef] =
-      collectWhile((nextByte == CASEDEF || nextByte == SHARED) && currentAddr != end) {
-        if (nextByte == SHARED) {
-          readByte()
-          forkAt(readAddr()).readCase()(ctx.fresh.setNewScope)
-        }
-        else readCase()(ctx.fresh.setNewScope)
-      }
+      collectWhile(nextUnsharedTag == CASEDEF && currentAddr != end) { readCase()(ctx.fresh.setNewScope) }
 
     def readCase()(implicit ctx: Context): CaseDef = {
       val start = currentAddr