protobuf: mark unlikely branch as so

Don't leave things to chance. We generally mark likely/unlikely
branches on hot paths as such.

PiperOrigin-RevId: 561288632

php/ext/google/protobuf/php-upb.c

@@ -1,6 +1,8 @@
 /* Amalgamated source file */
 #include "php-upb.h"
 
+test-xyz
+
 /*
  * This is where we define internal portability macros used across upb.
  *

ruby/ext/google/protobuf_c/ruby-upb.c

@@ -1,6 +1,8 @@
 /* Amalgamated source file */
 #include "ruby-upb.h"
 
+test-xyz
+
 /*
  * This is where we define internal portability macros used across upb.
  *

src/google/protobuf/serial_arena.h

@@ -133,7 +133,7 @@ class PROTOBUF_EXPORT SerialArena {
     // the pattern we are looking for.
     const size_t index = absl::bit_width(size - 1) - 4;
 
-    if (index >= cached_block_length_) return nullptr;
+    if (PROTOBUF_PREDICT_FALSE(index >= cached_block_length_)) return nullptr;
     auto& cached_head = cached_blocks_[index];
     if (cached_head == nullptr) return nullptr;