Unnecessary sign-extension for `LeadingZeroCount(UInt64)`

```cs
public static class LeadingZeroCount {
public static nint M1(ulong mask) {
return BitOperations.LeadingZeroCount(mask);
}

public static nint M2(ulong mask) {
return (nint)ulong.LeadingZeroCount(mask);
}
}

public static class TrailingZeroCount {
public static nint M1(ulong mask) {
return BitOperations.TrailingZeroCount(mask);
}

public static nint M2(ulong mask) {
return (nint)ulong.TrailingZeroCount(mask);
}
}

public static class PopCount {
public static nint M1(ulong mask) {
return BitOperations.PopCount(mask);
}

public static nint M2(ulong mask) {
return (nint)ulong.PopCount(mask);
}
}
```

```asm
// coreclr trunk-20250917+116db00b333e27b70b6e97c93bcfb304ca4245ab

LeadingZeroCount:M1(ulong):nint (FullOpts):
xor eax, eax
lzcnt rax, rdi
cdqe
ret

LeadingZeroCount:M2(ulong):nint (FullOpts):
xor eax, eax
lzcnt rax, rdi
ret

TrailingZeroCount:M1(ulong):nint (FullOpts):
xor eax, eax
tzcnt rax, rdi
cdqe
ret

TrailingZeroCount:M2(ulong):nint (FullOpts):
xor eax, eax
tzcnt rax, rdi
ret

PopCount:M1(ulong):nint (FullOpts):
xor eax, eax
popcnt rax, rdi
cdqe
ret

PopCount:M2(ulong):nint (FullOpts):
xor eax, eax
popcnt rax, rdi
ret
```

[godbolt.org](https://csharp.godbolt.org/z/M99qvqMGa)