Move size assertions for mir::syntax types into the same file

A redundant size assertion for `StatementKind` was added in #122937, because
the existing assertion was in a different file.

This patch cleans that up, and also moves the `TerminatorKind` assertion into
the same file where it belongs, to avoid the same thing happening again.

compiler/rustc_middle/src/mir/syntax.rs

@@ -1463,5 +1463,6 @@ mod size_asserts {
     static_assert_size!(PlaceElem<'_>, 24);
     static_assert_size!(Rvalue<'_>, 40);
     static_assert_size!(StatementKind<'_>, 16);
+    static_assert_size!(TerminatorKind<'_>, 96);
     // tidy-alphabetical-end
 }