rustc_query_system: simplify QueryCache::iter

[tgnottingham]

Minor cleanup to reduce a small amount of complexity and code bloat.
Reduces the number of mono items in rustc_query_impl by 15%.

[rust-highfive]

r? @varkor

(rust-highfive has picked a reviewer for you, use r? to override)

[tgnottingham]

@rustbot label T-compiler

[Mark-Simulacrum]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

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

[bors]

⌛ Trying commit 86bb1f6e46da2bd3af3636b3c0117a3de7016bb3 with merge 3050f42c7040dd83f083b829ab12fb12cace445e...

[tgnottingham]

@Mark-Simulacrum -- out of curiosity, is there any way to cancel the try build? I'm about to push another change.

Edit: looks like it cancels automatically on force push or something. Nevermind! :)

[tgnottingham]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

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

[bors]

⌛ Trying commit adcbe49 with merge 1c5d00c1311dca4f3bf740115a22c59a6446125b...

[bors]

☀️ Try build successful - checks-actions
Build commit: 1c5d00c1311dca4f3bf740115a22c59a6446125b (1c5d00c1311dca4f3bf740115a22c59a6446125b)

[rust-timer]

Queued 1c5d00c1311dca4f3bf740115a22c59a6446125b with parent b3e19a2, future comparison URL.

[rust-timer]

Finished benchmarking try commit (1c5d00c1311dca4f3bf740115a22c59a6446125b): comparison url.

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. Please note that if the perf results are neutral, you should likely undo the rollup=never given below by specifying rollup- to bors.

Importantly, though, if the results of this run are non-neutral do not roll this PR up -- it will mask other regressions or improvements in the roll up.

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

[cjgillot]

Good catch!
r? @cjgillot
@bors r+

[bors]

📌 Commit adcbe49 has been approved by cjgillot

[bors]

⌛ Testing commit adcbe49 with merge 56f74c5...

[bors]

☀️ Test successful - checks-actions
Approved by: cjgillot
Pushing 56f74c5 to master...