[V1][Minor] Minor simplification for get_computed_blocks (#16139)

Signed-off-by: Woosuk Kwon <woosuk.kwon@berkeley.edu>

vllm/v1/core/kv_cache_manager.py

@@ -126,7 +126,10 @@ class KVCacheManager:
             self.req_to_block_hashes[request.request_id] = block_hashes
 
         self.prefix_cache_stats.requests += 1
-        if request.sampling_params.prompt_logprobs is None:
+        # When the request requires prompt logprobs, we skip prefix caching.
+        if request.sampling_params.prompt_logprobs is not None:
+            return [], 0
+
         if len(block_hashes) * self.block_size == request.num_tokens:
             # When prompt length is divisible by the block size and all
             # blocks are cached, we need to recompute the last token. This
@@ -142,22 +145,20 @@ class KVCacheManager:
 
         computed_blocks = (
             self.specialized_manager.find_longest_cache_hit(block_hashes))
+        self.prefix_cache_stats.queries += len(block_hashes)
+        self.prefix_cache_stats.hits += len(computed_blocks)
 
         if last_block_hash is not None:
             # Add back the last block hash if it was removed.
+            # NOTE: Because block_hashes is cached in req_to_block_hashes,
+            # we shouldn't modify it directly.
             block_hashes.append(last_block_hash)
 
-            self.prefix_cache_stats.queries += len(block_hashes)
-            self.prefix_cache_stats.hits += len(computed_blocks)
-
         # NOTE(woosuk): Since incomplete blocks are not eligible for
         # sharing, `num_computed_tokens` is always a multiple of
         # `block_size`.
         num_computed_tokens = len(computed_blocks) * self.block_size
         return computed_blocks, num_computed_tokens
-        else:
-            # Skip cache hits for prompt logprobs
-            return [], 0
 
     def allocate_slots(
         self,