Auto merge of #108794 - nnethercote:avoid-unnecessary-hashing, r=cjgillot

Avoid unnecessary hashing

I noticed some stable hashing being done in a non-incremental build. It turns out that some of this is necessary to compute the crate hash, but some of it is not. Removing the unnecessary hashing is a perf win.

r? `@cjgillot`

Author: bors