Merge BuildReducedGraphVisitor into DefPathVisitor

[oli-obk]

These two visitors run right after each other on the same immutable AST. There's also a hash map for transferring the TyCtxtFeed created in the def collector to the BRG when it visits the same items. There are possibly more avenues for sharing logic, but I want to keep this PR simple.

only opening for perf runs for now. I'm still investigating how to ensure that future changes don't introduce subtle bugs by forgetting that def collection and reduced graph building are one pass now

Best reviewed commit-by-commit. I took a lot of care for making the individual changes reviewable, but all the Merge * commits aren't able to compile libcore until the last one.

[oli-obk]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 5d54692 (5d5469299abfae0527f2b94b434252c5a94a80a3, parent: 49b6ac01d6f4c3da812039ae846407a20961aa4c)

[rust-timer]

Finished benchmarking commit (5d54692): comparison URL.

Overall result: ✅ improvements - no action needed

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.

@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
Regressions ❌ (secondary)	0.1%	[0.1%, 0.1%]	1
Improvements ✅ (primary)	-0.3%	[-0.5%, -0.1%]	19
Improvements ✅ (secondary)	-0.3%	[-0.5%, -0.1%]	15
All ❌✅ (primary)	-0.3%	[-0.5%, -0.1%]	19

Max RSS (memory usage)

Results (primary 1.9%, secondary 2.0%)

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

	mean	range	count
Regressions ❌ (primary)	1.9%	[1.4%, 2.3%]	3
Regressions ❌ (secondary)	3.0%	[1.0%, 4.5%]	7
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-5.0%	[-5.0%, -5.0%]	1
All ❌✅ (primary)	1.9%	[1.4%, 2.3%]	3

Cycles

Results (secondary -0.8%)

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)	2.2%	[2.2%, 2.2%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.7%	[-3.7%, -3.7%]	1
All ❌✅ (primary)	-	-	0

Binary size

Results (primary 0.1%, secondary 0.1%)

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

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.0%, 0.1%]	63
Regressions ❌ (secondary)	0.1%	[0.0%, 0.1%]	24
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[0.0%, 0.1%]	63

Bootstrap: 487.434s -> 486.262s (-0.24%)
Artifact size: 395.10 MiB -> 395.19 MiB (0.02%)

[rust-bors]

☔ The latest upstream changes (presumably #153838) made this pull request unmergeable. Please resolve the merge conflicts.