Update array.fs
Update array.fs

*copied from https://github.com/fsharp/fsharp/pull/533*

There may be a missing length check on exists, as without hte length check the cost of a comparison and a `call` is incurred.

Adding the simple len > 0 check before the loop appears (in the basic cases I have tested) to near triple performance.
```
let f arr = 
    Array.exists (fun e -> e = true) arr

for i in 0..60000000 do f ([||]:bool []) ;; // 1.8 seconds

let f2 arr = 
    if Array.length arr > 0 then
        Array.exists (fun e -> e = true) arr
    else 
        false

for i in 0..60000000 do f2 ([||]:bool []) ;; // 0.430 seconds
```

Note: this is something I have found an isolated instance of, it may be appropriate to ensure that such checks (if they are required, I can only speak for my use case) are consistant accross other modules, such as Seq\List.
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