* Use maximum inline capacity available for `SmallVec<VRegIndex>` in `SpillSet`
We were using 2, which is the maximum for 32-bit architectures, but on 64-bit
architectures we can get 4 inline elements without growing the size of the
`SmallVec`.
This is a statistically significant speed up, but is so small that our
formatting of floats truncates it (so less than 1%).
```
compilation :: instructions-retired :: benchmarks/bz2/benchmark.wasm
Δ = 3360297.85 ± 40136.18 (confidence = 99%)
more-inline-capacity.so is 1.00x to 1.00x faster than main.so!
[945563401 945906690.73 946043245] main.so
[942192473 942546392.88 942729104] more-inline-capacity.so
compilation :: instructions-retired :: benchmarks/pulldown-cmark/benchmark.wasm
Δ = 1780540.13 ± 39362.84 (confidence = 99%)
more-inline-capacity.so is 1.00x to 1.00x faster than main.so!
[1544990595 1545359408.41 1545626251] main.so
[1543269057 1543578868.28 1543851201] more-inline-capacity.so
compilation :: instructions-retired :: benchmarks/spidermonkey/benchmark.wasm
Δ = 36577153.54 ± 243753.54 (confidence = 99%)
more-inline-capacity.so is 1.00x to 1.00x faster than main.so!
[33956158997 33957780594.50 33959538220] main.so
[33919762415 33921203440.96 33923023358] more-inline-capacity.so
```
* Use a `const fn` to calculate number of inline elements
b41b1f9a3c