Add note on regression being caused by some undoing of a performance gained in one particular perf run

Author: rylev

triage/2021-03-23.md

@@ -18,6 +18,7 @@ Implement (but don't use) valtree and refactor in preparation of use [#82936](ht
 Use TrustedRandomAccess for in-place iterators where possible [#79846](https://github.com/rust-lang/rust/issues/79846)
 - Moderate regression in [instruction counts](https://perf.rust-lang.org/compare.html?start=97663b6690689379aa0493deb494dfe14627c46b&end=35385770ae1ea86a911cc44ac43f856831e44b26&stat=instructions:u) (up to 1.5% on `full` builds of `deep-vector-debug`)
 - This is a change in the standard library that seems to only impact one benchmark: `deep-vector-debug` full compilation. It looks to be impacting `typeck`, but I'm not sure why this would be. It's also possible that it's noise.
+- There's also a possibility that this has some strange interaction with the performance gained in [#83360](https://github.com/rust-lang/rust/issues/83360).
 
 #### Improvements