Re-implement likely/unlikely with #[cold]

Since rust-lang/rust#120370 `#[cold]`
affects branch weights in the backend (again?). Re-implement
`likely` and `unlikely` in terms of a `#[cold]` function.

You can verify that the LLVM IR in [this godbolt link](https://godbolt.org/z/n3v7hsMqK)
contains a `branch_weights` metadata.

These were removed in rust-lang@d677fd4.

Author: DaniPopes

src/util.rs

@@ -1,10 +1,34 @@
-// FIXME: Branch prediction hint. This is currently only available on nightly
-// but it consistently improves performance by 10-15%.
-#[cfg(not(feature = "nightly"))]
-pub(crate) use core::convert::{identity as likely, identity as unlikely};
+// FIXME: Replace with `core::hint::{likely, unlikely}` once they are stable.
 #[cfg(feature = "nightly")]
 pub(crate) use core::intrinsics::{likely, unlikely};
 
+#[cfg(not(feature = "nightly"))]
+#[inline(always)]
+#[cold]
+fn cold_path() {}
+
+#[cfg(not(feature = "nightly"))]
+#[inline(always)]
+pub(crate) fn likely(b: bool) -> bool {
+    if b {
+        true
+    } else {
+        cold_path();
+        false
+    }
+}
+
+#[cfg(not(feature = "nightly"))]
+#[inline(always)]
+pub(crate) fn unlikely(b: bool) -> bool {
+    if b {
+        cold_path();
+        true
+    } else {
+        false
+    }
+}
+
 // FIXME: use strict provenance functions once they are stable.
 // Implement it with a transmute for now.
 #[inline(always)]