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?

compiler/rustc_mir/src/lib.rs

@@ -27,6 +27,7 @@ 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]