Skip computing extra liveness info if borrow_set is empty

[jackh726]

r? lqd

[rustbot]

Failed to set assignee to lqd: req=https://api.github.com/repos/rust-lang/rust/issues/161252/assignees (x-github-request-id: BE17:280341:1AC9DD:1D33F5:6A8349F1): {"message": "No server is currently available to service your request. Sorry about that. Please try resubmitting your request and contact us if the problem persists."}

Note: Only org members with at least the repository "read" role, users with write permissions, or people who have commented on the PR may be assigned.

[jackh726]

@bors try @rust-timer queue

[jackh726]

Disclosure: An LLM was used to dig into the icu4x (and other) regressions, and identified that that and other cases have no loans - and so that this is a candidate fix.

[rust-bors]

☀️ Try build successful (CI)
Build commit: 4b205ca (4b205caca087d3a42150f9866c94ad2cad745472)
Base parent: 6d656b1 (6d656b1efca82491a110b476ac9bbe712628ccb6)

[rust-timer]

Finished benchmarking commit (4b205ca): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never rustc-perf
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.1%	[0.0%, 0.2%]	5
Improvements ✅ (primary)	-0.6%	[-1.5%, -0.1%]	15
Improvements ✅ (secondary)	-0.3%	[-0.8%, -0.1%]	16
All ❌✅ (primary)	-0.6%	[-1.5%, -0.1%]	15

Max RSS (memory usage)

Results (primary 1.3%, secondary 1.2%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	1.3%	[0.5%, 2.5%]	5
Regressions ❌ (secondary)	3.2%	[3.2%, 3.2%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.8%	[-0.8%, -0.8%]	1
All ❌✅ (primary)	1.3%	[0.5%, 2.5%]	5

Cycles

This perf run didn't have relevant results for this metric.

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 468.493s -> 459.261s (-1.97%)
Artifact size: 398.93 MiB -> 398.95 MiB (0.01%)