The macros LLM and LLM_IF_ENABLED behave identically but are slightly different from a performance standpoint. For consistency all allocs and frees should use the same one. Measure the cost of a cook on our large internal project and remove LLM_IF_ENABLED if it is not useful or convert all allocs and frees to LLM_IF_ENABLED if it is.
They behave identically because OnLowLevelFree and OnLowLevelAlloc, like all LLM functions (maybe a few exceptions) early exit if bIsDisabled.
LLM_IF_ENABLED was added in CL 4114133 as a performance improvement along with some other changes to bIsEnabled. Other things have changed since then so it's possible the LLM_IF_ENABLED macro is no longer useful for performance. The only reason it might be useful is to avoid the cost of the function call.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-200297 in the post.