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/back/link.rs

@@ -174,9 +174,8 @@ pub fn each_linked_rlib(
             _ => {}
         }
     }
-    let fmts = match fmts {
-        Some(f) => f,
-        None => return Err("could not find formats for rlibs".to_string()),
+    let Some(fmts) = fmts else {
+        return Err("could not find formats for rlibs".to_string());
     };
     for &cnum in crates {
         match fmts.get(cnum.as_usize() - 1) {

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)]

compiler/rustc_codegen_ssa/src/mir/operand.rs

@@ -343,9 +343,7 @@ impl<'a, 'tcx, V: CodegenObject> OperandValue<V> {
             .unwrap_or_else(|| bug!("indirect_dest has non-pointer type: {:?}", indirect_dest))
             .ty;
 
-        let (llptr, llextra) = if let OperandValue::Ref(llptr, Some(llextra), _) = self {
-            (llptr, llextra)
-        } else {
+        let OperandValue::Ref(llptr, Some(llextra), _) = self else {
             bug!("store_unsized called with a sized value")
         };