Fix uninhabited enum branching pass

[ogoffart]

when the discriminant is taken with some projection.

[rust-highfive]

r? @jackh726

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

[ogoffart]

The context behind this PR is that I wanted to experiment with another optimisation on enums, and wanted to use this pass as a model. But I realized it did not match for most of the enums, so I needed to fix that first.

[jackh726]

I don't know enough about this code to review this myself, but this also seems small. Let's r? @wesleywiser though.

[wesleywiser]

Let's check performance before merging:

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

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

[bors]

⌛ Trying commit 5bece28 with merge dc41536a2e921160a1eeec16d307b49046e400a0...

[bors]

☀️ Try build successful - checks-actions
Build commit: dc41536a2e921160a1eeec16d307b49046e400a0 (dc41536a2e921160a1eeec16d307b49046e400a0)

[rust-timer]

Queued dc41536a2e921160a1eeec16d307b49046e400a0 with parent d4e7cb3, future comparison URL.

[rust-timer]

Finished benchmarking try commit (dc41536a2e921160a1eeec16d307b49046e400a0): comparison url.

Summary: This benchmark run did not return any significant changes.

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

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 -perf-regression

[rustbot]

Error: Label perf-regression can only be set by Rust team members

Please let @rust-lang/release know if you're having trouble with this bot.

[wesleywiser]

cc @rust-lang/release rust-timer can't seem to set labels anymore

[wesleywiser]

@bors r+ rollup=never

[bors]

📌 Commit 5bece28 has been approved by wesleywiser

[bors]

⌛ Testing commit 5bece28 with merge f726dbe...

[bors]

☀️ Test successful - checks-actions
Approved by: wesleywiser
Pushing f726dbe to master...