Move likely/unlikely argument outside of invisible unsafe block

The previous `likely!`/`unlikely!` macros were unsound because it
permits the caller's expr to contain arbitrary unsafe code.

    pub fn huh() -> bool {
        likely!(std::ptr::read(&() as *const () as *const bool))
    }

Before: compiles cleanly.
After:

    error[E0133]: call to unsafe function is unsafe and requires unsafe function or block
       |
    70 |     likely!(std::ptr::read(&() as *const () as *const bool))
       |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ call to unsafe function
       |
       = note: consult the function's documentation for information on how to avoid undefined behavior

compiler/rustc_data_structures/src/lib.rs

@@ -47,9 +47,9 @@ pub fn cold_path<F: FnOnce() -> R, R>(f: F) -> R {
 #[macro_export]
 macro_rules! likely {
     ($e:expr) => {
-        #[allow(unused_unsafe)]
-        {
-            unsafe { std::intrinsics::likely($e) }
+        match $e {
+            #[allow(unused_unsafe)]
+            e => unsafe { std::intrinsics::likely(e) },
         }
     };
 }
@@ -57,9 +57,9 @@ macro_rules! likely {
 #[macro_export]
 macro_rules! unlikely {
     ($e:expr) => {
-        #[allow(unused_unsafe)]
-        {
-            unsafe { std::intrinsics::unlikely($e) }
+        match $e {
+            #[allow(unused_unsafe)]
+            e => unsafe { std::intrinsics::unlikely(e) },
         }
     };
 }