compiler: avoid crashing on invalid non-integer array length

compiler: avoid crashing on invalid non-integer array length

    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