`RCS1124` may add unnecessary cast for nullable value types

[RenderMichael]

For the following code:

internal struct S;

internal static S? M()
{
    S? i = new S();
    return i;
}

RCS1124 is emitted, to inline the i variable. However, the fixer results in this:

internal struct S;

internal static S? M()
{
    return (S?)new S();
}

This unnecessary cast does not need to be emitted.

It's worth noting that a non-nullable local doesn't have this problem:

internal struct S;

internal static S? M()
{
    S i = new S();
    return i;
}

->

internal struct S;

internal static S? M()
{
    return new S();
}

(there's also no analyzer to remove this unnecessary cast, even Roslyn's IDE0004 doesn't trigger, but that may be another issue)