3D P2P could use some constant folding

Following up to #73 (closed), there's one pow in the computation of the scaling constant at the beginning of the kernel that gets generated for the same case as described in #73 (closed). perf top -z -a -p PID reveals that that is pointlessly eating at least 14% CPU. (where I'm guessing that pow is implement as exp(a*log(b)))

67.81%  p2p_from_csr.so   [.] _pocl_launcher_p2p_from_csr
7.21%  libm-2.24.so      [.] __ieee754_ilogb
7.04%  libm-2.24.so      [.] __ldexp
5.55%  libm-2.24.so      [.] __scalbn
3.70%  libm-2.24.so      [.] __ilogb
2.91%  libc-2.24.so      [.] __isinf
2.40%  p2p_from_csr.so   [.] ilogb@plt
1.14%  p2p_from_csr.so   [.] isnan@plt
1.08%  p2p_from_csr.so   [.] ldexp@plt
0.72%  p2p_from_csr.so   [.] isinf@plt
0.35%  libc-2.24.so      [.] __isnan

cc @mattwala