Coalesce branches

Move a bunch of branches together into one if block, for easier reading.

Resolve comments

Attempt to make some branches unreachable [tmp]

Revert unreachable branches

compiler/rustc_trait_selection/src/lib.rs

@@ -21,6 +21,7 @@
 #![feature(label_break_value)]
 #![feature(let_chains)]
 #![feature(let_else)]
+#![feature(if_let_guard)]
 #![feature(never_type)]
 #![recursion_limit = "512"] // For rustdoc