Small CPU optimization in InlineSkipList::Insert() (facebook#12975)

Summary:
reuse decode key in more places to avoid decoding length prefixed key x->Key().

Pull Request resolved: facebook#12975

Test Plan:
ran benchmarks simultaneously for "before" and "after"
* fillseq:
```
(for I in $(seq 1 50); do ./db_bench --benchmarks=fillseq --disable_auto_compactions=1 --min_write_buffer_number_to_merge=100 --max_write_buffer_number=1000  --write_buffer_size=268435456 --num=5000000 --seed=1723056275 --disable_wal=1 2>&1 | grep "fillseq"
done;) | awk '{ t += $5; c++; print } END { printf ("%9.3f\n", 1.0 * t / c) }';

before: 1483191
after: 1490555 (+0.5%)
```

* fillrandom:
```
(for I in $(seq 1 2); do ./db_bench_imain --benchmarks=fillrandom --disable_auto_compactions=1 --min_write_buffer_number_to_merge=100 --max_write_buffer_number=1000  --write_buffer_size=268435456 --num=2500000 --seed=1723056275 --disable_wal=1 2>&1 | grep "fillrandom"

before: 255463
after: 256128 (+0.26%)
```

Reviewed By: anand1976

Differential Revision: D61835340

Pulled By: cbi42

fbshipit-source-id: 70345510720e348bacd51269acb5d2dd5a62bf0a

memtable/inlineskiplist.h

@@ -969,12 +969,12 @@ bool InlineSkipList<Comparator>::Insert(const char* key, Splice* splice,
       while (true) {
         // Checking for duplicate keys on the level 0 is sufficient
         if (UNLIKELY(i == 0 && splice->next_[i] != nullptr &&
-                     compare_(x->Key(), splice->next_[i]->Key()) >= 0)) {
+                     compare_(splice->next_[i]->Key(), key_decoded) <= 0)) {
           // duplicate key
           return false;
         }
         if (UNLIKELY(i == 0 && splice->prev_[i] != head_ &&
-                     compare_(splice->prev_[i]->Key(), x->Key()) >= 0)) {
+                     compare_(splice->prev_[i]->Key(), key_decoded) >= 0)) {
           // duplicate key
           return false;
         }
@@ -1012,12 +1012,12 @@ bool InlineSkipList<Comparator>::Insert(const char* key, Splice* splice,
       }
       // Checking for duplicate keys on the level 0 is sufficient
       if (UNLIKELY(i == 0 && splice->next_[i] != nullptr &&
-                   compare_(x->Key(), splice->next_[i]->Key()) >= 0)) {
+                   compare_(splice->next_[i]->Key(), key_decoded) <= 0)) {
         // duplicate key
         return false;
       }
       if (UNLIKELY(i == 0 && splice->prev_[i] != head_ &&
-                   compare_(splice->prev_[i]->Key(), x->Key()) >= 0)) {
+                   compare_(splice->prev_[i]->Key(), key_decoded) >= 0)) {
         // duplicate key
         return false;
       }