Fix compound assignments in liveness check

[chenyukang]

Fixes #154457

r? @ghost

Let's do a perf run, I assume there will be some trivial regression on perf:

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 6d9873d (6d9873daccf14745d652a8e0c93dbaba6bbc066e, parent: b5e038d7158c1af55a646027fdacf5ecd7c783c7)

[chenyukang]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 31dbf74 (31dbf74d42d6559a579fa7b09f1d9311edfd1bcb, parent: cced03bfd61a304243a34504618ecec86c17063f)

[rust-timer]

Finished benchmarking commit (31dbf74): comparison URL.

Overall result: ❌ regressions - 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
@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)	0.2%	[0.1%, 0.3%]	15
Regressions ❌ (secondary)	0.2%	[0.1%, 0.3%]	7
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.2%	[0.1%, 0.3%]	15

Max RSS (memory usage)

Results (primary 2.4%, secondary 0.9%)

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

	mean	range	count
Regressions ❌ (primary)	5.2%	[3.2%, 7.4%]	3
Regressions ❌ (secondary)	0.9%	[0.9%, 0.9%]	1
Improvements ✅ (primary)	-1.7%	[-2.5%, -1.0%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.4%	[-2.5%, 7.4%]	5

Cycles

Results (primary 2.5%, secondary 2.7%)

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

	mean	range	count
Regressions ❌ (primary)	2.5%	[2.5%, 2.5%]	1
Regressions ❌ (secondary)	2.7%	[2.7%, 2.7%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.5%	[2.5%, 2.5%]	1

Binary size

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

Bootstrap: 511.528s -> 511.622s (0.02%)
Artifact size: 400.68 MiB -> 400.66 MiB (-0.01%)

[rust-bors]

☔ The latest upstream changes (presumably #157121) made this pull request unmergeable. Please resolve the merge conflicts.