Rollup merge of #53313 - llogiq:two-small-improvements, r=estebank

Two small improvements

In `librustc_apfloat/ieee.rs`, use the iterator.[r]find methods to simplify the code. In `libserialize/json.rs`, make use of the fact that `Vec.last` on an empty `Vec` returns `None` to simplify the code to a single match.

diff --cc src/libserialize/json.rs
Simple merge