x64: Add shuffle specialization for palignr (#5999)

* x64: Add `shuffle` specialization for `palignr`

This commit adds specializations for the `palignr` instruction to the
x64 backend to specialize some more patterns of byte shuffles.

* Fix tests

cranelift/codegen/src/isa/x64/lower/isle.rs

@@ -1117,6 +1117,16 @@ impl Context for IsleContext<'_, '_, MInst, X64Backend> {
             None
         }
     }
+
+    fn palignr_imm_from_immediate(&mut self, imm: Immediate) -> Option<u8> {
+        let bytes = self.lower_ctx.get_immediate_data(imm).as_slice();
+
+        if bytes.windows(2).all(|a| a[0] + 1 == a[1]) {
+            Some(bytes[0])
+        } else {
+            None
+        }
+    }
 }
 
 impl IsleContext<'_, '_, MInst, X64Backend> {