Auto merge of #50106 - nnethercote:nearest_common_ancestor, r=nikomatsakis

Speed up `nearest_common_ancestor`.

`nearest_common_ancestor` can be made faster.

Here are all the benchmarks where one of the measurements improved by at least 1%.
```
clap-rs-check
	avg: -4.5%	min: -8.8%	max: -0.3%
clap-rs
	avg: -2.6%	min: -4.5%	max: 0.5%
script-servo
	avg: -1.7%	min: -3.6%	max: 0.0%
regression-31157
	avg: -1.5%	min: -2.6%	max: -0.4%
hyper
	avg: -1.2%	min: -2.5%	max: -0.0%
piston-image
	avg: -1.6%	min: -2.5%	max: 0.1%
regex
	avg: -1.2%	min: -2.2%	max: 0.0%
issue-46449
	avg: -1.8%	min: -2.1%	max: -0.7%
crates.io
	avg: -1.2%	min: -2.1%	max: 0.0%
hyper-check
	avg: -1.0%	min: -2.1%	max: -0.1%
clap-rs-opt
	avg: -1.4%	min: -2.0%	max: -0.3%
piston-image-check
	avg: -1.2%	min: -1.9%	max: -0.1%
regex-check
	avg: -0.5%	min: -1.8%	max: -0.1%
syn
	avg: -1.1%	min: -1.7%	max: -0.1%
tokio-webpush-simple-check
	avg: -1.1%	min: -1.6%	max: -0.3%
tokio-webpush-simple
	avg: -1.2%	min: -1.6%	max: -0.0%
helloworld-check
	avg: -1.4%	min: -1.6%	max: -1.2%
deeply-nested
	avg: -1.2%	min: -1.4%	max: -0.8%
encoding-check
	avg: -0.8%	min: -1.3%	max: -0.3%
unify-linearly-check
	avg: -1.0%	min: -1.3%	max: -0.8%
script-servo-check
	avg: -0.6%	min: -1.3%	max: 0.0%
regression-31157-check
	avg: -0.9%	min: -1.2%	max: -0.7%
script-servo-opt
	avg: -0.5%	min: -1.2%	max: 0.1%
deeply-nested-check
	avg: -0.8%	min: -1.2%	max: -0.7%
encoding
	avg: -0.7%	min: -1.1%	max: -0.3%
issue-46449-check
	avg: -0.9%	min: -1.1%	max: -0.6%
parser-check
	avg: -0.9%	min: -1.1%	max: -0.8%
html5ever
	avg: -0.5%	min: -1.0%	max: -0.0%
```