TOKYO—SoftBank’s planned sale of the British semiconductor and software design company Arm to U.S. chip maker Nvidia has fallen through, but the Japanese technology investor immediately turned bullish on taking it public. SoftBank Group Corp. said Tuesday it plans an initial public offering of Arm after the intended sale to Nvidia failed due to regulatory problems. It said the IPO would come sometime in the fiscal year ending in March 2023. Chief Executive Masayoshi Son acknowledged he was disappointed but wasted no time in shifting to an aggressive sales pitch for Arm in its preparation to go public in the United States, likely on the Nasdaq exchange. “Arm is back. Rather just being back, it’s really going to grow explosively,” Son told reporters. He said “a golden time” was coming because of Arm’s “architecture,” or technology for semiconductors, already widely used in cell phones and adapted by net giants like …