Update root cause for 140633

Author: Mark-Simulacrum

triage/2025-05-04.md

@@ -150,4 +150,4 @@ Rollup of 7 pull requests [#140633](https://github.com/rust-lang/rust/pull/14063
 | Improvements ✅ <br /> (secondary) | -     | -              | 0     |
 | All ❌✅ (primary)                 | 1.2%  | [-1.5%, 13.4%] | 78    |
 
-Still investigating the root cause as of writing (see comments on the PR).
+Per investigation on the PR, the root cause is [#139675](https://github.com/rust-lang/rust/pull/139675). Per discussion [here](https://github.com/rust-lang/rust/pull/140652) we expect that this will be mostly resolved in our next LLVM bump, but is likely an acceptable regression in the meantime (primarily to very short compiles) for the feature in question.