Fix VM's sametype impl to work for generics/typedescs (#19073)

* Fix vm's sametype implementation to properly handle generics and typedescs

* actually fixed sametype + have test

* added comments and removed unsafe code

compiler/types.nim

@@ -1128,6 +1128,11 @@ proc sameTypeAux(x, y: PType, c: var TSameTypeClosure): bool =
       a = a.skipTypes({tyDistinct, tyGenericInst})
       if a.kind != b.kind: return false
 
+  #[
+    The following code should not run in the case either side is an generic alias,
+    but it's not presently possible to distinguish the genericinsts from aliases of
+    objects ie `type A[T] = SomeObject`
+  ]#
   # this is required by tunique_type but makes no sense really:
   if tyDistinct notin {x.kind, y.kind} and x.kind == tyGenericInst and IgnoreTupleFields notin c.flags:
     let

compiler/vm.nim

@@ -1044,7 +1044,8 @@ proc rawExecute(c: PCtx, start: int, tos: PStackFrame): TFullReg =
                                      strictSymEquality=true))
     of opcSameNodeType:
       decodeBC(rkInt)
-      regs[ra].intVal = ord(regs[rb].node.typ.sameTypeOrNil regs[rc].node.typ)
+      regs[ra].intVal = ord(regs[rb].node.typ.sameTypeOrNil(regs[rc].node.typ, {ExactTypeDescValues, ExactGenericParams}))
+      # The types should exactly match which is why we pass `{ExactTypeDescValues..ExactGcSafety}`.
     of opcXor:
       decodeBC(rkInt)
       regs[ra].intVal = ord(regs[rb].intVal != regs[rc].intVal)