Simplify option-iterator flattening in the compiler

[cuviper]

In many cases where we are flattening option::IntoIter<I>, we can use
Option::into_flat_iter() instead for a simpler implementation. A few
other cases have more direct constructs available.

[cuviper]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 118fe6f (118fe6fbecfcc44a3d5617a8c99ec3edeb72d5f6)
Base parent: f46ec52 (f46ec5218fe7829ac18323b5ee0b409a63169f27)

[rust-timer]

Finished benchmarking commit (118fe6f): comparison URL.

Overall result: ✅ improvements - 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

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%	[-0.3%, -0.3%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.3%	[-0.3%, -0.3%]	2

Max RSS (memory usage)

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

Cycles

Results (primary 2.3%, secondary -0.9%)

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

	mean	range	count
Regressions ❌ (primary)	2.3%	[2.3%, 2.3%]	1
Regressions ❌ (secondary)	2.1%	[2.1%, 2.1%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.9%	[-3.9%, -3.9%]	1
All ❌✅ (primary)	2.3%	[2.3%, 2.3%]	1

Binary size

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

Bootstrap: 485.565s -> 488.649s (0.64%)
Artifact size: 393.29 MiB -> 393.15 MiB (-0.04%)

[cuviper]

Of course I hoped for more of a perf improvement, but at least it's not bad. :)

[rustbot]

Some changes occurred to the core trait solver

cc @rust-lang/initiative-trait-system-refactor

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

Some changes occurred in coverage instrumentation.

cc @Zalathar

[rustbot]

r? @nnethercote

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The reviewer was selected based on:

• Owners of files modified in this PR: compiler
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