perf: Avoid freshening the same predicate twice per selection

[xmakro]

candidate_from_obligation freshens a predicate that push_stack already freshened. The two results can only differ if the predicate contains type or const inference variables. If it has none, we reuse stack.fresh_trait_pred and skip the second freshening.

[Kobzol]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: e0160e1 (e0160e1f3fe1659f4d274a0552c202f82c5d2d01)
Base parent: c9ff496 (c9ff496891c278ad660bc0ab85c1f0b72059464a)

[rust-timer]

Finished benchmarking commit (e0160e1): comparison URL.

Overall result: ✅ improvements - no action needed

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.

@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
Improvements ✅ (primary)	-0.3%	[-0.4%, -0.2%]	14
Improvements ✅ (secondary)	-1.2%	[-3.4%, -0.2%]	22
All ❌✅ (primary)	-0.3%	[-0.4%, -0.2%]	14

Max RSS (memory usage)

Results (primary -0.6%, secondary -0.3%)

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)	2.3%	[2.3%, 2.3%]	1
Improvements ✅ (primary)	-0.6%	[-0.6%, -0.6%]	1
Improvements ✅ (secondary)	-0.6%	[-0.7%, -0.4%]	10
All ❌✅ (primary)	-0.6%	[-0.6%, -0.6%]	1

Cycles

Results (primary -0.0%, secondary 7.2%)

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

	mean	range	count
Regressions ❌ (primary)	1.0%	[0.5%, 2.3%]	10
Regressions ❌ (secondary)	8.7%	[0.4%, 20.1%]	18
Improvements ✅ (primary)	-3.2%	[-6.6%, -0.6%]	3
Improvements ✅ (secondary)	-2.0%	[-2.9%, -0.6%]	3
All ❌✅ (primary)	-0.0%	[-6.6%, 2.3%]	13

Binary size

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

Bootstrap: 489.838s -> 489.173s (-0.14%)
Artifact size: 390.28 MiB -> 390.28 MiB (0.00%)