Shrink TyKind::FnPtr.

By splitting the `FnSig` within `TyKind::FnPtr` into `FnSigTys` and
`FnHeader`, which can be packed more efficiently. This reduces the size
of the hot `TyKind` type from 32 bytes to 24 bytes on 64-bit platforms.
This reduces peak memory usage by a few percent on some benchmarks. It
also reduces cache misses and page faults similarly, though this doesn't
translate to clear cycles or wall-time improvements on CI.

compiler/rustc_codegen_cranelift/src/value_and_place.rs

@@ -872,7 +872,7 @@ pub(crate) fn assert_assignable<'tcx>(
         (ty::Ref(_, a, _), ty::RawPtr(b, _)) | (ty::RawPtr(a, _), ty::Ref(_, b, _)) => {
             assert_assignable(fx, *a, *b, limit - 1);
         }
-        (ty::FnPtr(_), ty::FnPtr(_)) => {
+        (ty::FnPtr(..), ty::FnPtr(..)) => {
             let from_sig = fx.tcx.normalize_erasing_late_bound_regions(
                 ParamEnv::reveal_all(),
                 from_ty.fn_sig(fx.tcx),