Use pflow variable in place of p->flow to prevent reloading.

 In SigMatchSignatures, the value p->flow doesn't change, but GCC can't
figure that out, so it reloads p->flow many times during the function.
When p->flow is loaded into the variable pflow once at the start of the
function, the compile then doesn't need to reload it.

Also addresses Victor's review comments to mark pflow as a constant pointer.

Passes both regression tests:
https://buildbot.suricata-ids.org/builders/ken-tilera/builds/68
https://buildbot.suricata-ids.org/builders/ken-tilera-pcap/builds/5