I'm not sure if it's the first occcurrence of this idea, but one of the main ideas of "SSA is functional programming" by Andrew Appel in 1998 is almost precisel

I'm not sure if it's the first occcurrence of this idea, but one of the main ideas of "SSA is functional programming" by Andrew Appel in 1998 is almost precisely this: https://www.cs.princeton.edu/~appel/papers/ssafun.pdf https://www.cs.princeton.edu/~appel/papers/ssafun.pdf (TL;DR: see the left column on page 2)