Auto merge of #61857 - Centril:typeck-extract-expr, r=oli-obk

typeck: extract expr type-checking to expr.rs + refactor check_expr_kind

In this PR we:

- Extract out the bulk of the expression type checking logic from `check/mod.rs` into a new file `check/expr.rs`.

- Refactor `fn check_expr_kind` into several smaller functions.

More functions should probably be moved but I think this is a reasonable start.

r? @oli-obk
cc @eddyb