perf: Inline some hot functions in FnDecl

[teor2345]

This PR tries to fix a perf regression in 1 primary benchmark introduced by #157699, by inlining some hot functions.

Regression details: #157699 (comment)

[panstromek]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: c53cee2 (c53cee2a16c43d810b2fb1c86492488e270ac5ff, parent: 9d862dd2c17b2dc90e9f623bb9279a8c4090b311)

[rust-timer]

Finished benchmarking commit (c53cee2): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

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

Max RSS (memory usage)

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

Cycles

Results (primary -3.4%, 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)	-	-	0
Regressions ❌ (secondary)	3.6%	[2.3%, 5.6%]	5
Improvements ✅ (primary)	-3.4%	[-3.4%, -3.4%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-3.4%	[-3.4%, -3.4%]	1

Binary size

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

Bootstrap: 519.36s -> 518.453s (-0.17%)
Artifact size: 400.90 MiB -> 400.92 MiB (0.01%)

[teor2345]

This gives slightly more negative than positive results, and the average is a slight negative, so I don't think it's worth doing.

[teor2345]

@rustbot label +F-splat