Avoid redundant UB check in RangeFrom slice indexing

[saethlin]

I noticed this while picking through the IR we generate for #134938. I think we just forgot to apply this trick to RangeFrom?

[scottmcm]

@bors r+ rollup=iffy

[bors]

📌 Commit 4f1b945 has been approved by scottmcm

It is now in the queue for this repository.

[saethlin]

@rust-timer build 55b1ddf

[rust-timer]

Finished benchmarking commit (55b1ddf): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@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)	0.4%	[0.3%, 0.6%]	2
Regressions ❌ (secondary)	0.2%	[0.0%, 0.6%]	3
Improvements ✅ (primary)	-0.3%	[-0.3%, -0.2%]	2
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.1%]	2
All ❌✅ (primary)	0.1%	[-0.3%, 0.6%]	4

Max RSS (memory usage)

Results (primary 2.6%, secondary -3.6%)

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

	mean	range	count
Regressions ❌ (primary)	4.8%	[3.7%, 6.0%]	2
Regressions ❌ (secondary)	2.3%	[2.1%, 2.4%]	2
Improvements ✅ (primary)	-1.9%	[-1.9%, -1.9%]	1
Improvements ✅ (secondary)	-4.7%	[-5.8%, -3.6%]	11
All ❌✅ (primary)	2.6%	[-1.9%, 6.0%]	3

Cycles

Results (secondary -0.4%)

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

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.5%	[2.1%, 4.2%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.3%	[-7.4%, -1.5%]	4
All ❌✅ (primary)	-	-	0

Binary size

Results (primary -0.1%, secondary -0.2%)

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

	mean	range	count
Regressions ❌ (primary)	0.2%	[0.0%, 0.6%]	10
Regressions ❌ (secondary)	1.7%	[0.1%, 3.2%]	2
Improvements ✅ (primary)	-0.2%	[-0.6%, -0.0%]	38
Improvements ✅ (secondary)	-0.3%	[-0.4%, -0.0%]	55
All ❌✅ (primary)	-0.1%	[-0.6%, 0.6%]	48

Bootstrap: 475.635s -> 475.478s (-0.03%)
Artifact size: 388.48 MiB -> 388.13 MiB (-0.09%)