Auto merge of #75058 - ssomers:btree_cleanup_insert_2, r=Mark-Simulacrum
Auto merge of #75058 - ssomers:btree_cleanup_insert_2, r=Mark-Simulacrum

Clarify reuse of a BTreeMap insert support function and treat split support likewise

r? @Mark-Simulacrum

aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust