Zulfikar Ali Bhutto was the first Pakistani Prime Minister to visit the Soviet Union in March 1972. While Liaquat Ali Khan received an invitation in 1949, he never visited the Soviet Union during his regime, and President Ayub Khan, not the Prime Minister, visited the Soviet Union in 1965.
Bhutto made his first official state visit to Moscow as Prime Minister to normalize relations after the 1971 war. He had also visited earlier in 1965, but at that time he was serving as the Foreign Minister under President Ayub Khan.
Quick info: Liaquat Ali Khan visited the USA instead of the USSR in 1950.