Adopt let_else across the compiler

This performs a substitution of code following the pattern:

let <id> = if let <pat> = ... { identity } else { ... : ! };

To simplify it to:

let <pat> = ... { identity } else { ... : ! };

By adopting the let_else feature.

compiler/rustc_monomorphize/src/lib.rs

@@ -2,6 +2,7 @@
 #![feature(bool_to_option)]
 #![feature(crate_visibility_modifier)]
 #![feature(control_flow_enum)]
+#![feature(let_else)]
 #![feature(in_band_lifetimes)]
 #![recursion_limit = "256"]