Fix instruction size test for Rust 1.65.0 (#5188)

Looks like Rust generously shrank our `enum` in 1.65.0, so update the
test assertion to pass CI.

cranelift/codegen/src/isa/x64/inst/mod.rs

@@ -48,7 +48,7 @@ pub struct CallInfo {
 fn inst_size_test() {
     // This test will help with unintentionally growing the size
     // of the Inst enum.
-    assert_eq!(48, std::mem::size_of::<Inst>());
+    assert_eq!(40, std::mem::size_of::<Inst>());
 }
 
 pub(crate) fn low32_will_sign_extend_to_64(x: u64) -> bool {