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
