compiler: eliminate bound checks in append expression

compiler: eliminate bound checks in append expression

    The compiler generates two array index expressions when lowering
    an append expression. Currently they generate bound checks.
    Bound checks are not necessary in this case, as we know the slice
    has, or will grow to, enough length and capacity. Eliminate them.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/166817

From-SVN: r269699