compiler: avoid call duplication during order eval.

compiler: avoid call duplication during order eval.

    During the compiler's order_evaluations() pass, duplicate
    call statements were being created and inserted into block
    statement lists in situations where there were calls returning
    multiple arguments. These duplicates were benign in that they
    were ignored during backend code generation, however they
    did cause duplicate calls in AST dumps. Tweak the order
    evaluation code to handle this case better and avoid inserting
    these duplicates.

    Fixes golang/go#17237

    Reviewed-on: https://go-review.googlesource.com/29857

From-SVN: r240559