topdown/copypropagation: avoid circular reference (#3637)

When running copy propagation on

    x = input[x]

the AST transformation would run into a stack overflow: the removedEqs
map would return a reference containing the variable we've started with.
Evaluating that reference, we'd run into an infinite loop when
transforming its consituents:

    x => input[x] => {input, x} => input => x

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>