Auto merge of #128812 - nnethercote:shrink-TyKind-FnPtr, r=compiler-errors

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.

r? `@compiler-errors`

compiler/rustc_ty_utils/src/layout.rs

@@ -184,7 +184,7 @@ fn layout_of_uncached<'tcx>(
         ty::Int(ity) => scalar(Int(Integer::from_int_ty(dl, ity), true)),
         ty::Uint(ity) => scalar(Int(Integer::from_uint_ty(dl, ity), false)),
         ty::Float(fty) => scalar(Float(Float::from_float_ty(fty))),
-        ty::FnPtr(_) => {
+        ty::FnPtr(..) => {
             let mut ptr = scalar_unit(Pointer(dl.instruction_address_space));
             ptr.valid_range_mut().start = 1;
             tcx.mk_layout(LayoutS::scalar(cx, ptr))