Fix VSO bug 405852.

Fix VSO bug 405852.

This bug was due to a mismatch between the semantics of the FP modulus
operator as specified by ECMA-335 and as implemented by value numbering.

ECMA-335 requires the following behavior for FP modulus:
- If the divisor is 0 or the dividend is not finite, the result is NaN
- If the divisor is not finite and is not NaN, the result is the
  dividend.
Value numbering, however, simply used `fmod`, which does not have these
semantics. This change implements the required semantics in VN.