Optimize _double_var

- Save 1 _fe_negate
- Prefer operations on local variables
- Reorder code somewhat to group inline function calls together

I am definitely looking for others to test this one. I measure up to 1% improvement for bench_verify, but of the three things I listed above, the "reorder code" measures as the most significant, so it's possible I have only optimized for my machine/compiler somehow.