Unnecessary sign-extension for some never-negative expressions

```cs
nint M1(uint mask) {
return BitOperations.TrailingZeroCount(mask) / sizeof(ushort);
}

nint M2(uint mask) {
return (nint)(uint.TrailingZeroCount(mask) / sizeof(ushort));
}
```

```asm
C:M1(uint):nint:this (FullOpts):
xor eax, eax
tzcnt eax, esi
shr eax, 1
cdqe
ret

C:M2(uint):nint:this (FullOpts):
xor eax, eax
tzcnt eax, esi
shr eax, 1
ret
```

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