fix(deadcode): account for impure paths inside for statements

[vigneshshanmugam]

• fix simplify's loop rewriting incorrectly removes preceding variable declarations #824

[j-f1]

What happens if there’s a side-effectful expression in the update/condition parts of the loop?

[vigneshshanmugam]

Shouldn't be a problem since we are doing this transform only if test statement is pure. The update path won't be executed at all in these cases.