[libc] Fix leader calculation when done in wave64 mode

Summary:
Wave 64 mode touches the upper limit of this, which had an off-by-one
error. This caused it to return the same leader which gave an invalid
view of memory.

Author: jhuber6

libc/src/__support/GPU/allocator.cpp

@@ -156,7 +156,7 @@ static inline constexpr uint32_t get_start_index(uint32_t chunk_size) {
 
 // Returns the id of the lane below this one that acts as its leader.
 static inline uint32_t get_leader_id(uint64_t ballot, uint32_t id) {
-  uint64_t mask = id < BITS_IN_DWORD ? ~0ull << (id + 1) : 0;
+  uint64_t mask = id < BITS_IN_DWORD - 1 ? ~0ull << (id + 1) : 0;
   return BITS_IN_DWORD - cpp::countl_zero(ballot & ~mask) - 1;
 }