update AST-to-HIR lowering examples for conditionals and loops

- `for` loops now use two `match`es for all of their bindings. I'm not
sure this is the most helpful way of conveying that, but it's about as
informative as before while staying brief.

- `while let` and `if let` don't use `match`; they use `let` expressions
in their conditions. Since `if let` no longer has significantly
different desugaring and having a whole bullet point for `while` would
feel redundant with `for`, I've removed those examples.

Author: dianne

src/doc/rustc-dev-guide/src/hir/lowering.md

@@ -7,10 +7,8 @@ of such structures include but are not limited to
 
 * Parenthesis
     * Removed without replacement, the tree structure makes order explicit
-* `for` loops and `while (let)` loops
-    * Converted to `loop` + `match` and some `let` bindings
-* `if let`
-    * Converted to `match`
+* `for` loops
+    * Converted to `match` + `loop` + `match`
 * Universal `impl Trait`
     * Converted to generic arguments
       (but with some flags, to know that the user didn't write them)