Cheaper threadfence for gfx942 in postPeer [1/N]: enable for single node allreduce (#1766)

Boosts single node bfloat16 allreduce performance by up to 20% for some data sizes and provides gating with the RCCL_GFX942_CHEAP_FENCE_OFF environment variable

src/include/comm.h

@@ -488,6 +488,7 @@ struct ncclComm {
   int node;
   int nNodes;
   int rcclUseOneSlice; // RCCL: true if this comm is using one slice per primitive
+  int gfx942CheapFenceOff; // RCCL: true if gfx942 cheap fence is disabled
   int localRank;
   int localRanks;
   int maxLocalRanks;