Move assertion out of loop for better performance

Move assertion out of loop for better performance

This is related to bug 508. The debug build was taking much longer than the
production build to compute the result. The issue was an assertion in a loop in
nonClausalSimplify(). By moving the assertion outside of the loop, the debug
build is now roughly an order of magnitude slower than the production build
(instead of two orders of magnitude), which seems to be roughly in line with
the difference for other benchmarks:

Debug version before change:
- Bug (minified version): 1065.6s
- regress3/friedman_n6_i4.smt: 6.9s
- regress2/uflia-error0.smt2: 6.3s
- regress2/fuzz_2.smt: 2.3s

Debug version after change:
- Bug (minified version): 131.4s
- regress3/friedman_n6_i4.smt: 6.7s
- regress2/uflia-error0.smt2: 6.2s
- regress2/fuzz_2.smt: 1.9s

Production version:
- Bug (minified version): 18.8s
- regress3/friedman_n6_i4.smt: 0.8s
- regress2/uflia-error0.smt2: 0.8s
- regress2/fuzz_2.smt: 0.2s