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_gcc/src/type_of.rs

@@ -213,9 +213,8 @@ impl<'tcx> LayoutGccExt<'tcx> for TyAndLayout<'tcx> {
                 // NOTE: we cannot remove this match like in the LLVM codegen because the call
                 // to fn_ptr_backend_type handle the on-stack attribute.
                 // TODO(antoyo): find a less hackish way to hande the on-stack attribute.
-                ty::FnPtr(sig) => {
-                    cx.fn_ptr_backend_type(cx.fn_abi_of_fn_ptr(sig, ty::List::empty()))
-                }
+                ty::FnPtr(sig_tys, hdr) => cx
+                    .fn_ptr_backend_type(cx.fn_abi_of_fn_ptr(sig_tys.with(hdr), ty::List::empty())),
                 _ => self.scalar_gcc_type_at(cx, scalar, Size::ZERO),
             };
             cx.scalar_types.borrow_mut().insert(self.ty, ty);