Avoid doing mempool specific leak search activities if there are no mempools

Avoid doing mempool specific leak search activities if there are no mempools

For most memcheck users, no mempools are used, but the leak search logic
was doing in any case special handling, leading to useless work such as
sorting again an already sorted array and making a copy of an array without
modifying it.

This slightly optimises the perf reg tests of memcheck.
perl perf/vg_perf --tools=memcheck --vg=. --vg=../trunk_untouched perf
-- Running  tests in perf ----------------------------------------------
-- bigcode1 --
bigcode1 .         :0.08s  me: 3.0s (38.1x, -----)
bigcode1 trunk_untouched:0.08s  me: 3.1s (38.6x, -1.3%)
-- bigcode2 --
bigcode2 .         :0.07s  me: 7.4s (105.9x, -----)
bigcode2 trunk_untouched:0.07s  me: 7.5s (107.4x, -1.5%)
-- bz2 --
bz2      .         :0.40s  me: 5.2s (12.9x, -----)
bz2      trunk_untouched:0.40s  me: 5.4s (13.6x, -5.0%)
-- fbench --
fbench   .         :0.15s  me: 2.8s (18.8x, -----)
fbench   trunk_untouched:0.15s  me: 2.9s (19.0x, -1.1%)
-- ffbench --
ffbench  .         :0.16s  me: 2.7s (16.8x, -----)
ffbench  trunk_untouched:0.16s  me: 2.7s (17.1x, -1.9%)
-- heap --
heap     .         :0.06s  me: 4.0s (66.5x, -----)
heap     trunk_untouched:0.06s  me: 4.1s (68.7x, -3.3%)
-- heap_pdb4 --
heap_pdb4 .         :0.07s  me: 6.2s (89.1x, -----)
heap_pdb4 trunk_untouched:0.07s  me: 6.6s (94.9x, -6.4%)
-- many-loss-records --
many-loss-records .         :0.01s  me: 1.2s (122.0x, -----)
many-loss-records trunk_untouched:0.01s  me: 1.2s (125.0x, -2.5%)
-- many-xpts --
many-xpts .         :0.03s  me: 1.2s (41.7x, -----)
many-xpts trunk_untouched:0.03s  me: 1.3s (43.7x, -4.8%)
-- memrw --
memrw    .         :0.06s  me: 1.2s (19.8x, -----)
memrw    trunk_untouched:0.06s  me: 1.2s (20.2x, -1.7%)
-- sarp --
sarp     .         :0.02s  me: 1.8s (91.5x, -----)
sarp     trunk_untouched:0.02s  me: 2.1s (103.5x,-13.1%)
-- tinycc --
tinycc   .         :0.11s  me: 7.1s (64.4x, -----)
tinycc   trunk_untouched:0.11s  me: 7.1s (64.3x,  0.1%)
-- Finished tests in perf ----------------------------------------------

== 12 programs, 24 timings =================