Optimize the codegen for `Span::from_expansion`
See https://godbolt.org/z/bq65Y6bc4 for the difference. the new version is less than half the number of instructions.
Also tried fully writing the function by hand:
```rust
sp.ctxt_or_parent_or_marker != 0
&& (
sp.len_with_tag_or_marker == BASE_LEN_INTERNED_MARKER
|| sp.len_with_tag_or_marker & PARENT_TAG == 0
)
```
But that was no better than this PR's current use of `match_span_kind`.
a2ae171b97