Rollup merge of #70254 - matthiaskrgr:cl4ppy, r=Centril

couple more clippy fixes (let_and_return, if_same_then_else)

* summarize if-else-code with identical blocks (clippy::if_same_then_else)
* don't create variable bindings just to return the bound value immediately (clippy::let_and_return)

diff --cc src/librustc/hir/map/mod.rs
Simple merge

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

diff --cc src/librustc_parse/lexer/mod.rs
Simple merge