Count leading zeros of uint32 in F#

CountLeadingZeros.fs

let inline leading_zeros_of_ls4b_of_uint32 x =
    match x with
    | 0b0000u -> 4
    | 0b0001u -> 3
    | 0b0010u -> 2
    | 0b0011u -> 2
    | 0b0100u -> 1
    | 0b0101u -> 1
    | 0b0110u -> 1
    | 0b0111u -> 1
    | _ -> 0

let leading_zeros_of_uint32 input =
    let mutable n = 0;
    let mutable reg = input
    if (reg >>> 16) = 0u then n <- n + 16; reg <- reg <<< 16
    if (reg >>> 24) = 0u then n <- n +  8; reg <- reg <<<  8
    if (reg >>> 28) = 0u then n <- n +  4; reg <- reg <<<  4
    n + leading_zeros_of_ls4b_of_uint32 (reg >>> 28)