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

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

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

compiler/rustc_middle/src/mir/mod.rs

@@ -1823,9 +1823,7 @@ mod size_asserts {
     static_assert_size!(LocalDecl<'_>, 40);
     static_assert_size!(SourceScopeData<'_>, 64);
     static_assert_size!(Statement<'_>, 32);
-    static_assert_size!(StatementKind<'_>, 16);
     static_assert_size!(Terminator<'_>, 112);
-    static_assert_size!(TerminatorKind<'_>, 96);
     static_assert_size!(VarDebugInfo<'_>, 88);
     // tidy-alphabetical-end
 }

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
 }