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.

src/test/codegen/array-clone.rs

@@ -0,0 +1,15 @@
+// compile-flags: -O
+
+#![crate_type = "lib"]
+
+// CHECK-LABEL: @array_clone
+#[no_mangle]
+pub fn array_clone(a: &[u8; 2]) -> [u8; 2] {
+    // CHECK-NOT: getelementptr
+    // CHECK-NOT: load i8
+    // CHECK-NOT: zext
+    // CHECK-NOT: shl
+    // CHECK: load i16
+    // CHECK-NEXT: ret i16
+    a.clone()
+}