Minor Type.IsPrimitive improvement

[EgorBo]

Re-use branch-free version in RuntimeHelpers instead.
It makes Type.IsPrimitive property a bit faster and removes duplicated code:

[Benchmark]
[Arguments(typeof(string))]
[Arguments(typeof(Program))]
[Arguments(typeof(sbyte))]
[Arguments(typeof(int))]
[Arguments(typeof(UIntPtr))]
public bool IsPrimitive(Type type) => type.IsPrimitive;

Before:
|      Method |    type |     Mean |     Error |    StdDev |
|------------ |-------- |---------:|----------:|----------:|
| IsPrimitive | Program | 2.022 ns | 0.0047 ns | 0.0042 ns |
| IsPrimitive |   Int32 | 2.396 ns | 0.0038 ns | 0.0032 ns |
| IsPrimitive |   SByte | 2.406 ns | 0.0053 ns | 0.0047 ns |
| IsPrimitive |  String | 1.986 ns | 0.0007 ns | 0.0006 ns |
| IsPrimitive | UIntPtr | 2.192 ns | 0.0015 ns | 0.0012 ns |

After:
|      Method |    type |     Mean |     Error |    StdDev |
|------------ |-------- |---------:|----------:|----------:|
| IsPrimitive | Program | 1.714 ns | 0.0062 ns | 0.0051 ns |
| IsPrimitive |   Int32 | 1.927 ns | 0.0047 ns | 0.0041 ns |
| IsPrimitive |   SByte | 1.947 ns | 0.0060 ns | 0.0053 ns |
| IsPrimitive |  String | 1.737 ns | 0.0186 ns | 0.0174 ns |
| IsPrimitive | UIntPtr | 1.923 ns | 0.0023 ns | 0.0020 ns |

[jkotas]

Thanks