Optimize str.substr reduction (#3595)

Optimize str.substr reduction (#3595)

This commit optimizes the `str.substr` reduction by replacing the
if-then-else term for the length of the suffix `len(sk2) = ite(len(s) >=
n+m, len(s) - (n + m), 0)` with `(len(sk2) = len(s) - (n + m) v
len(sk2) = 0) ^ len(skt) <= m`. Experiments have shown that the latter
encoding is more efficient.