(lshift_double): Don't report arithmetic overflow, since the
C standard doesn't seem to require it here.
(force_fit_type): Accept new parameter specifying whether a signed
overflow has already occurred.  Yield overflow indication.
All callers changed.
(neg_double): Don't report overflow when -0 yields 0.
(const_binop): Propagate operand overflow for warnings.
Don't report overflow when -1 + 1 yields 0.

From-SVN: r3401