Tweak the array type checking code to avoid crashing on array types
    whose length expressions are explicit non-integer types (for example,
    "float64(10)"). If such constructs are seen, issue an "invalid array
    bound" error.
    
    Fixes golang/go#13486.
    
    Reviewed-on: https://go-review.googlesource.com/91975

From-SVN: r259900