compiler: don't incorrectly evaluate range variable

compiler: don't incorrectly evaluate range variable

    The language spec says that in `for i = range x`, in which there is no
    second iteration variable, if len(x) is constant, then x is not
    evaluated.  This only matters when x is an expression that panics but
    whose type is an array type; in such a case, we should not evaluate x,
    since len of any array type is a constant.

    Fixes golang/go#22313

    Reviewed-on: https://go-review.googlesource.com/91555

From-SVN: r257330