Performance of LLVM 12 vs LLVM 11, take 2

commmit

Filtered for unodb::db:

• micro_benchmark_key_prefix: 2% speedup (unpredictable_get_shared_length) to 6% speedup (unpredictable_prepend_key_prefix)
• micro_benchmark_n4: 3% slowdown (n4_full_scan/100) to 2% speedup (full_n4_random_deletes/65534)

Filtered for unodb::olc_db:

• micro_benchmark_key_prefix: 2% slowdown (unpredictable_get_shared_length) to 1% speedup (unpredictable_cut_key_prefix)
• micro_benchmark_n4: 6% slowdown (full_n4_random_deletes/100) to 0% speedup (full_n4_random_insert/100)