Field checks for everybody (#8957)

* Field checks for JS backend

* Clean nkCall nodes with no arguments

Generating a nkEmpty in place of no arguments makes no sense form the
AST point of view and also trips up the VM codegen.

* Field checks for VM backend

* Test case for #6612

This patchset fixes #6612

* Add test case for LHS double evaluation

* Prevent LHS double-eval for JS backend

* Prevent double evaluation in VM backend

tests/js/t6612.nim

@@ -0,0 +1,24 @@
+discard """
+  action: "run"
+"""
+
+proc fillWith(sq: var seq[int], n: int, unused: string) =
+  sq = @[n]
+
+type
+  Object = object of RootObj
+    case hasNums: bool
+    of true:
+      numbers: seq[int]
+    of false:
+      discard
+    always: seq[int]
+
+var obj = Object(hasNums: true)
+
+obj.always.fillWith(5, "unused")
+doAssert obj.always == @[5]
+
+obj.numbers.fillWith(3, "unused")
+doAssert obj.numbers == @[3]
+doAssert obj.always == @[5]