Update our range assumes to the format that LLVM prefers

[scottmcm]

I found out in llvm/llvm-project#123278 (comment) that the way I started emitting the assumes in #109993 was suboptimal, and as seen in that LLVM issue the way we're doing it -- with two assumes sometimes -- can at times lead to CVP/SCCP not realize what's happening because one of them turns into a ne instead of conveying a range.

So this updates how it's emitted from

assume( x >= LOW );
assume( x <= HIGH );

or

// (for ranges that wrap the range)
assume( (x <= LOW) | (x >= HIGH) );

to

assume( (x - LOW) <= (HIGH - LOW) );

so that we don't need multiple icmps nor multiple assumes for a single value, and both wrappping and non-wrapping ranges emit the same shape.

(And we don't bother emitting the subtraction if LOW is zero, since that's trivial for us to check too.)

[rustbot]

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[scottmcm]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 6fe8200 with merge 37b0240...

[bors]

☀️ Try build successful - checks-actions
Build commit: 37b0240 (37b0240d0706c13aff3886502fd74a0ddc2db9b1)

[rust-timer]

Finished benchmarking commit (37b0240): comparison URL.

Overall result: ❌✅ regressions and improvements - no action needed

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

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

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.5%	[0.4%, 0.6%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.3%	[-0.3%, -0.3%]	2
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary -2.2%, secondary -0.8%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-2.2%	[-3.2%, -1.2%]	2
Improvements ✅ (secondary)	-0.8%	[-0.8%, -0.8%]	1
All ❌✅ (primary)	-2.2%	[-3.2%, -1.2%]	2

Cycles

Results (primary 2.0%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	2.0%	[2.0%, 2.0%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.0%	[2.0%, 2.0%]	1

Binary size

Results (secondary -0.0%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.0%	[-0.0%, -0.0%]	1
All ❌✅ (primary)	-	-	0

Bootstrap: 765.445s -> 765.133s (-0.04%)
Artifact size: 326.00 MiB -> 326.04 MiB (0.01%)

[estebank]

The code changes look fine to me. I'm ok with landing this, if the perf results make sense to you.