By Arriana McLymore and Ross Kerber NEW YORK, Dec 18 - A union-aligned investment group sent letters to Amazon, Walmart and Alphabet on Wednesday, asking how U.S. President Donald Trump's immigration ​policies were impacting their finances and supply chains, according to the documents seen by Reuters. SOC Investment ‌Group owns less than 1% of each of the companies, which include some of the top recipients of H-1B visa petition approvals ‌for skilled foreign professionals. It wants the companies to show ...