compiler: fix evaluation order of LHS index expressions

compiler: fix evaluation order of LHS index expressions

    The spec says that when an index expression appears on the left hand
    side of an assignment, the operands should be evaluated. The
    gofrontend code was assuming that that only referred to the index
    operand. But discussion of https://golang.org/issue/23188 has
    clarified that this means both the slice/map/string operand and the
    index operand. Adjust the gofrontend code accordingly.

    Fixes golang/go#23188

    Reviewed-on: https://go-review.googlesource.com/123155

From-SVN: r262554