Optimize RefCell read borrowing

[luca-barbieri]

Instead of doing two comparisons we can do only one with a bit of cleverness.

LLVM currently can't do this optimization itself on x86-64.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @kennytm (or someone else) soon.

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[kennytm]

r? @RalfJung

Implication of this change is too subtle for me to decide :)

[nikic]

I've reported https://bugs.llvm.org/show_bug.cgi?id=42691 for the optimization failure.

[RalfJung]

Thanks, this looks right!

@bors r+

[bors]

📌 Commit 44c1650 has been approved by RalfJung

[Centril]

@bors rollup=never

[Centril]

@bors try

[bors]

🙅 Please do not try after a pull request has been r+ed. If you need to try, unapprove (r-) it first.

[bors]

⌛ Testing commit 44c1650 with merge 0e9b465...

[bors]

☀️ Test successful - checks-azure
Approved by: RalfJung
Pushing 0e9b465 to master...