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

In SigMatchSignatures, the value p->flow doens'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.

Passes both regression tests:
https://buildbot.suricata-ids.org/builders/ken-tilera/builds/67
https://buildbot.suricata-ids.org/builders/ken-tilera-pcap/builds/4