pmrep: optimize instance filtering
pmrep: optimize instance filtering

A rather trivial patch provides a nice win for sth like:

pmrep -i '.*firefox.*' proc

old:
all  30.10s user 1.51s system 25% cpu 2:02.40 total
-i   13.60s user 0.12s system 11% cpu 1:59.80 total

new:
all  31.11s user 1.53s system 26% cpu 2:02.59 total
-i    7.19s user 0.12s system  6% cpu 1:59.76 total

Note that we want to check this only when instances are
specified, otherwise we'd see:

all  41.55s user 1.66s system 35% cpu 2:02.76 total
-i    6.99s user 0.11s system  5% cpu 1:59.63 total

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp