Simplify some pattern-matches (#16402)

When those match an exact combinator on Option / Result.

Tool-aided by [comby-rust](https://github.com/huitseeker/comby-rust).

runtime/src/accounts_hash.rs

@@ -1861,11 +1861,10 @@ pub mod tests {
         // handle fanout^x -1, +0, +1 for a few 'x's
         const FANOUT: usize = 3;
         let mut hash_counts: Vec<_> = (1..6)
-            .map(|x| {
+            .flat_map(|x| {
                 let mark = FANOUT.pow(x);
                 vec![mark - 1, mark, mark + 1]
             })
-            .flatten()
             .collect();
 
         // saturate the test space for threshold to threshold + target

runtime/src/message_processor.rs

@@ -379,10 +379,11 @@ impl<'a> InvokeContext for ThisInvokeContext<'a> {
                 self.account_deps.iter().find(|(key, _)| key == pubkey)
             {
                 Some(account.clone())
-            } else if let Some(pre) = self.pre_accounts.iter().find(|pre| pre.key == *pubkey) {
-                Some(pre.account.clone())
             } else {
-                None
+                self.pre_accounts
+                    .iter()
+                    .find(|pre| pre.key == *pubkey)
+                    .map(|pre| pre.account.clone())
             }
         } else {
             if let Some(account) = self.pre_accounts.iter().find_map(|pre| {