Avoid memory allocation when removing dead blocks

[tmiasko]

Use reachable_as_bitset to reuse a bitset from the traversal rather
than allocating it seprately. Additionally check if there are any
unreachable blocks before proceeding.

[rust-highfive]

r? @matthewjasper

(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.

[bors]

⌛ Trying commit dc6ec35 with merge f4eaeac6891ab5e03034bb6f220380a7bd667d5c...

[bors]

☀️ Try build successful - checks-actions
Build commit: f4eaeac6891ab5e03034bb6f220380a7bd667d5c (f4eaeac6891ab5e03034bb6f220380a7bd667d5c)

[rust-timer]

Queued f4eaeac6891ab5e03034bb6f220380a7bd667d5c with parent 643a79a, future comparison URL.

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

[rust-timer]

Finished benchmarking try commit (f4eaeac6891ab5e03034bb6f220380a7bd667d5c): 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

[matthewjasper]

@bors r+

[bors]

📌 Commit dc6ec35 has been approved by matthewjasper

[bors]

⌛ Testing commit dc6ec35 with merge b122908...

[bors]

☀️ Test successful - checks-actions
Approved by: matthewjasper
Pushing b122908 to master...