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_mir_dataflow/src/lib.rs

@@ -6,6 +6,7 @@
 #![feature(exact_size_is_empty)]
 #![feature(in_band_lifetimes)]
 #![feature(iter_zip)]
+#![feature(let_else)]
 #![feature(min_specialization)]
 #![feature(once_cell)]
 #![feature(stmt_expr_attributes)]