Function try_verify_p2pkh_fast_path 

pub fn try_verify_p2pkh_fast_path(
    script_sig: &Vec<u8>,
    script_pubkey: &[u8],
    flags: u32,
    tx: &Transaction,
    input_index: usize,
    prevout_values: &[i64],
    prevout_script_pubkeys: &[&[u8]],
    block_height: Option<u64>,
    network: Network,
    precomputed_sighash_all: Option<[u8; 32]>,
    sighash_cache: Option<&Arc<Mutex<HashMap<SighashCacheKey, [u8; 32], FxBuildHasher>>>>,
    precomputed_p2pkh_hash: Option<[u8; 20]>,
) -> Option<Result<bool, ConsensusError>>

P2PKH fast-path. Returns Some(Ok(bool)) if script is P2PKH and we handled it; Returns None to fall back to full interpreter.