Simplify size/align_of_val<T: Sized> to size/align_of<T> instead

[scottmcm]

This is relevant to things like Box<[u8; 1024]> where the drop looks at the size_of_val (since obviously it might be DST in general) but where we don't actually need to do that since it's always that same value for the Sized type.

(Equivalent to #152681, but flipped in the rebase so it can land before #152641 instead of depending on it.)

[scottmcm]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[rust-bors]

☀️ Try build successful (CI)
Build commit: 85c1367 (85c13679ed60d98f876a8f50c0b7505fb1ea3260, parent: 139651428df86cf88443295542c12ea617cbb587)

[rust-timer]

Queued 85c1367 with parent 1396514, future comparison URL.
There are currently 2 preceding artifacts in the queue.
It will probably take at least ~3.0 hours until the benchmark run finishes.