Pretty print generator witness only in -Zverbose mode

[tmiasko]

In release build of deeply-nested-async benchmark the size of
no-opt.bc file is reduced from 46MB to 62kB.

Helps with #84873, where in one of reported test cases the size of no-opt.bc
file is reduced from 2.3GB to 799kB.

[rust-highfive]

r? @estebank

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

[tmiasko]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

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

[bors]

⌛ Trying commit 78b981fe115633ab7214445101473349cbdfc3ab with merge 3475950431a8883d7ca11420c9954459c70a0b95...

[bors]

☀️ Try build successful - checks-actions
Build commit: 3475950431a8883d7ca11420c9954459c70a0b95 (3475950431a8883d7ca11420c9954459c70a0b95)

[rust-timer]

Queued 3475950431a8883d7ca11420c9954459c70a0b95 with parent b84ffce, future comparison URL.

[rust-timer]

Finished benchmarking try commit (3475950431a8883d7ca11420c9954459c70a0b95): 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

[jackh726]

r? @jackh726
@bors r+

[bors]

📌 Commit 78b981fe115633ab7214445101473349cbdfc3ab has been approved by jackh726

[bors]

⌛ Testing commit 78b981fe115633ab7214445101473349cbdfc3ab with merge 1a3665a8aa94e4c017cb09ac7292dffb843a9469...

[bors]

💔 Test failed - checks-actions

[tmiasko]

Rebased and updated output of a test case added in #86215.

[jackh726]

@bors r+

[bors]

📌 Commit 06ca736 has been approved by jackh726

[bors]

⌛ Testing commit 06ca736 with merge 24bdc6d...

[bors]

☀️ Test successful - checks-actions
Approved by: jackh726
Pushing 24bdc6d to master...

[audunska]

This PR seems to have fixed my exponential compile times issue mentioned in #84873! 🎉

That is, compile times return to normal between nightly 2021-06-12 and 2021-06-13.