Fix arithmetic stability issue (#2817)

It turns out "simple_stmt" isn't that simple: it can contain multiple
statements separated by semicolons. Invisible parenthesis logic for
arithmetic expressions only looked at the first child of simple_stmt.
This causes instability in the presence of semicolons, since the next
run through the statement following the semicolon will be the first
child of another simple_stmt.

I believe this along with #2572 fix the known stability issues.