* 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
