I audited all the op1/op2 range for undefined values, but missed that an 
intervening calculation can also cause an undefined range in the middle 
of operator_plus::op1_range, and that is the passed to 
adjust_op1_for_overflow.   That routine also needs to check for 
undefined before asking for the type of the range.

Bootstrapped on x86_64-pc-linux-gnu with no regressions.  Pushed.

Andrew