Specialize array cloning for Copy types

Because after PR 86041, the optimizer no longer load-merges at the LLVM IR level, which might be part of the perf loss.  (I'll run perf and see if this makes a difference.)

Also I added a codegen test so this hopefully won't regress in future -- it passes on stable and with my change here, but not on the 2021-11-09 nightly.

library/core/src/array/mod.rs

@@ -339,9 +339,7 @@ impl<T: Copy, const N: usize> Copy for [T; N] {}
 impl<T: Clone, const N: usize> Clone for [T; N] {
     #[inline]
     fn clone(&self) -> Self {
-        // SAFETY: we know for certain that this iterator will yield exactly `N`
-        // items.
-        unsafe { collect_into_array_unchecked(&mut self.iter().cloned()) }
+        SpecArrayClone::clone(self)
     }
 
     #[inline]
@@ -350,6 +348,27 @@ impl<T: Clone, const N: usize> Clone for [T; N] {
     }
 }
 
+#[cfg(not(bootstrap))]
+trait SpecArrayClone: Clone {
+    fn clone<const N: usize>(array: &[Self; N]) -> [Self; N];
+}
+
+#[cfg(not(bootstrap))]
+impl<T: Clone> SpecArrayClone for T {
+    default fn clone<const N: usize>(array: &[T; N]) -> [T; N] {
+        // SAFETY: we know for certain that this iterator will yield exactly `N`
+        // items.
+        unsafe { collect_into_array_unchecked(&mut array.iter().cloned()) }
+    }
+}
+
+#[cfg(not(bootstrap))]
+impl<T: Copy> SpecArrayClone for T {
+    fn clone<const N: usize>(array: &[T; N]) -> [T; N] {
+        *array
+    }
+}
+
 // The Default impls cannot be done with const generics because `[T; 0]` doesn't
 // require Default to be implemented, and having different impl blocks for
 // different numbers isn't supported yet.