Auto merge of #128544 - compiler-errors:perf-warn_if_unreachable, r=f…

…mease

Check divergence value first before doing span operations in `warn_if_unreachable`

It's more expensive to extract the span's desugaring first rather than check the value of the divergence enum. For some reason I inverted these checks, probably for readability, but as a consequence I regressed perf:

rust-lang/rust#128443 (comment)

r? fmease