Micro-optimize Object#class

Since `BUILTIN_TYPE` and `RCLASS_SINGLETON_P` are both stored in
`RBasic.flags`, we can combine these two checks in a single bitmask.

This rely on `T_ICLASS` and `T_CLASS` not overlapping, and assume
`klass` is always either of these types.

Just combining the masks brings a small but consistent 1.08x speedup on the simple case benchmark.

```
compare-ruby: ruby 3.5.0dev (2025-08-30T01:45:42Z obj-class 01a57bd6cd) +YJIT +PRISM [arm64-darwin24]
built-ruby: ruby 3.5.0dev (2025-08-30T09:56:24Z obj-class 2685f8dbb4) +YJIT +PRISM [arm64-darwin24]

|           |compare-ruby|built-ruby|
|:----------|-----------:|---------:|
|obj        |     444.410|   478.895|
|           |           -|     1.08x|
|extended   |     135.139|   140.206|
|           |           -|     1.04x|
|singleton  |     165.155|   155.832|
|           |       1.06x|         -|
|immediate  |     380.103|   432.090|
|           |           -|     1.14x|
```

But with the RB_UNLIKELY compiler hint, it's much more significant, however
the singleton and enxtended cases are slowed down.
However we can assume the simple case is way more common than the other two.

```
compare-ruby: ruby 3.5.0dev (2025-08-30T01:45:42Z obj-class 01a57bd6cd) +YJIT +PRISM [arm64-darwin24]
built-ruby: ruby 3.5.0dev (2025-08-30T09:51:01Z obj-class 12d01a1b02) +YJIT +PRISM [arm64-darwin24]

|           |compare-ruby|built-ruby|
|:----------|-----------:|---------:|
|obj        |     444.951|   556.191|
|           |           -|     1.25x|
|extended   |     136.836|   113.871|
|           |       1.20x|         -|
|singleton  |     166.335|   167.747|
|           |           -|     1.01x|
|immediate  |     379.642|   509.515|
|           |           -|     1.34x|
```

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