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_smir/src/rustc_internal/internal.rs

@@ -131,7 +131,10 @@ impl RustcInternal for RigidTy {
             RigidTy::FnDef(def, args) => {
                 rustc_ty::TyKind::FnDef(def.0.internal(tables, tcx), args.internal(tables, tcx))
             }
-            RigidTy::FnPtr(sig) => rustc_ty::TyKind::FnPtr(sig.internal(tables, tcx)),
+            RigidTy::FnPtr(sig) => {
+                let (sig_tys, hdr) = sig.internal(tables, tcx).split();
+                rustc_ty::TyKind::FnPtr(sig_tys, hdr)
+            }
             RigidTy::Closure(def, args) => {
                 rustc_ty::TyKind::Closure(def.0.internal(tables, tcx), args.internal(tables, tcx))
             }