destructors: optimize more assignments into moves; also fixes #9294

tests/destructor/tprevent_assign2.nim

@@ -0,0 +1,48 @@
+discard """
+  errormsg: "'=' is not available for type <Foo>; requires a copy because it's not the last read of 'otherTree'"
+  line: 44
+"""
+
+type
+  Foo = object
+    x: int
+
+proc `=destroy`(f: var Foo) = f.x = 0
+proc `=`(a: var Foo; b: Foo) {.error.} # = a.x = b.x
+proc `=sink`(a: var Foo; b: Foo) = a.x = b.x
+
+proc createTree(x: int): Foo =
+  Foo(x: x)
+
+proc take2(a, b: sink Foo) =
+  echo a.x, " ", b.x
+
+proc allowThis() =
+  var otherTree: Foo
+  for i in 0..3:
+    while true:
+      #if i == 0:
+      otherTree = createTree(44)
+      case i
+      of 0:
+        echo otherTree
+        take2(createTree(34), otherTree)
+      of 1:
+        take2(createTree(34), otherTree)
+      else:
+        discard
+
+proc preventThis() =
+  var otherTree: Foo
+  for i in 0..3:
+    while true:
+      if i == 0:
+        otherTree = createTree(44)
+      case i
+      of 0:
+        echo otherTree
+        take2(createTree(34), otherTree)
+      of 1:
+        take2(createTree(34), otherTree)
+      else:
+        discard