Rollup merge of #76818 - hbina:dont_compile_regex_all_the_time, r=ecstatic-morse

Don't compile regex at every function call.

Use `SyncOnceCell` to only compile it once.
I believe this still adds some kind of locking mechanism?

Related issue: https://github.com/rust-lang/rust/issues/76817

compiler/rustc_mir/src/lib.rs

@@ -28,6 +28,7 @@ Rust MIR: a lowered representation of Rust.
 #![feature(trait_alias)]
 #![feature(option_expect_none)]
 #![feature(or_patterns)]
+#![feature(once_cell)]
 #![recursion_limit = "256"]
 
 #[macro_use]