Auto merge of #34638 - zackmdavis:if_let_over_none_empty_block_arm, r=jseyfried

prefer `if let` to match with `None => {}` arm in some places

This is a spiritual succesor to #34268 / 8531d581, in which we replaced a
number of matches of None to the unit value with `if let` conditionals
where it was judged that this made for clearer/simpler code (as would be
recommended by Manishearth/rust-clippy's `single_match` lint). The same
rationale applies to matches of None to the empty block.

----

r? @jseyfried

src/librustc_const_eval/check_match.rs

@@ -176,9 +176,8 @@ fn check_expr(cx: &mut MatchCheckCtxt, ex: &hir::Expr) {
 
                 // Second, if there is a guard on each arm, make sure it isn't
                 // assigning or borrowing anything mutably.
-                match arm.guard {
-                    Some(ref guard) => check_for_mutation_in_guard(cx, &guard),
-                    None => {}
+                if let Some(ref guard) = arm.guard {
+                    check_for_mutation_in_guard(cx, &guard);
                 }
             }