arc optimizations (#13325)

* scope based destructors
* handle 'or' and 'and' expressions properly, see the new test arc/tcontrolflow.nim
* make this branch mergable, logic is disabled for now

tests/arc/tcontrolflow.nim

@@ -0,0 +1,55 @@
+discard """
+  output: '''begin A
+elif
+destroyed
+end A
+begin false
+if
+destroyed
+end false
+begin true
+if
+end true
+'''
+  cmd: "nim c --gc:arc -d:danger $file"
+  disabled: "true"
+"""
+# we use the -d:danger switch to detect uninitialized stack
+# slots more reliably (there shouldn't be any, of course).
+
+# XXX Enable once scope based destruction works!
+
+type
+  Foo = object
+    id: int
+
+proc `=destroy`(x: var Foo) =
+  if x.id != 0:
+    echo "destroyed"
+    x.id = 0
+
+proc construct(): Foo = Foo(id: 3)
+
+proc elifIsEasy(cond: bool) =
+  echo "begin A"
+  if cond:
+    echo "if"
+  elif construct().id == 3:
+    echo "elif"
+  else:
+    echo "else"
+  echo "end A"
+
+elifIsEasy(false)
+
+
+proc orIsHard(cond: bool) =
+  echo "begin ", cond
+  if cond or construct().id == 3:
+    echo "if"
+  else:
+    echo "else"
+  echo "end ", cond
+
+orIsHard(false)
+orIsHard(true)