Add a cheap mode for `compute_missing_ctors`.
Add a cheap mode for `compute_missing_ctors`.

`compute_missing_ctors` is called a lot. It produces a vector, which can
be reasonably large (e.g. 100+ elements), but the vector is almost
always only checked for emptiness.

This commit changes `compute_missing_ctors` so it can be called in a
cheap way that just indicates if the vector would be empty. If
necessary, the function can subsequently be called in an expensive way
to compute the full vector.

This change reduces instruction counts for several benchmarks up to 2%.

graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust