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_codegen_ssa/src/lib.rs

@@ -3,6 +3,7 @@
 #![feature(box_patterns)]
 #![feature(try_blocks)]
 #![feature(in_band_lifetimes)]
+#![feature(let_else)]
 #![feature(once_cell)]
 #![feature(nll)]
 #![feature(associated_type_bounds)]