Auto merge of #33541 - eddyb:promote-only-temps, r=arielb1

mir: don't attempt to promote Unpromotable constant temps.

Fixes #33537. This was a non-problem in regular functions, but we also promote in `const fn`s.
There we always qualify temps so you can't depend on `Unpromotable` temps being `NOT_CONST`.

diff --cc src/librustc_mir/transform/qualify_consts.rs
Simple merge