Small mir-opt refactor

[scottmcm]

Hopefully-non-controversial changes from some not-ready-yet work that I'd figured I'd submit on their own.

[rust-highfive]

r? @michaelwoerister

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

[michaelwoerister]

r? rust-lang/mir-opt

[nagisa]

@bors r+

[bors]

📌 Commit b215a32 has been approved by nagisa

[bors]

⌛ Testing commit b215a32 with merge ecb65b0e170fc5275870c9d0fba7267a57cf35e5...

[bors]

☀️ Test successful - checks-actions
Approved by: nagisa
Pushing ecb65b0e170fc5275870c9d0fba7267a57cf35e5 to master...

[bors]

👀 Test was successful, but fast-forwarding failed: 422 Update is not a fast forward

[scottmcm]

Well, this passed, so might as well just put it in a
@bors rollup

but it probably also needs a
@bors retry

[rust-timer]

Finished benchmarking commit (ecb65b0e170fc5275870c9d0fba7267a57cf35e5): comparison url.

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

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

@rustbot label: -perf-regression

[bors]

⌛ Testing commit b215a32 with merge 0fb1c37...

[bors]

☀️ Test successful - checks-actions
Approved by: nagisa
Pushing 0fb1c37 to master...

[rust-timer]

Finished benchmarking commit (0fb1c37): comparison url.

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

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

@rustbot label: -perf-regression