sbyte array comparison doesn't work correctly with negative values

Whilst working on the `IComparable` (which includes the inequality operators) version of #5112, I found that the currently implementation of `sbyte[]` do not work correctly when one of the operands is negative.

#### Repro steps

let a = -1y
let b = 0y

printfn "%b" (a < b)
printfn "%b" ([a] < [b])
printfn "%b" ([|a|] < [|b|])

#### Expected behavior

true
true
true

#### Actual behavior

true
true
false

#### Known workarounds

None. Well could convert the array to a list and then do a comparison there (if you can).

#### Related information

This was found when I was creating the alternative implementation and then running against the test suite created in #577. So presumably this has been a bug dating back prior to the creation of that regression suite (Aug 9, 2015). So probably no urgency to fix, and when the next PR that I'm working on is done then it will be cleared up.