[Highlight] Performance improvements

[Tom de Vries <tdevries@suse.de>]

Hi,

I've been working on performance improvements for dwz, using a cc1
binary as my optimization vehicle.

Comparing the situation:
- before (commit 04a676d Add --devel-partition-dups-opt), and
- after (current master, commit e405c62 Add --devel-die-count-method
  {none,estimate})
I get the following results.

When avoiding running into the low-mem die-limit using -lnone, we get
~25% performance improvement, due to an improved hash function and an
improved hash table allocation strategy (without increasing peak memory
usage):
...
real:  mean:  7378.10  100.00%  stddev:  45.31
       mean:  5558.80   75.34%  stddev:  35.18
user:  mean:  7106.30  100.00%  stddev:  41.53
       mean:  5328.10   74.98%  stddev:  22.33
sys:   mean:   271.60  100.00%  stddev:  39.57
       mean:   230.00   84.68%  stddev:  40.45
...

And if we don't avoid running into the low-mem die-limit, we get ~38%
performance improvement:
...
real:  mean:  15084.80 100.00%  stddev:  44.53
       mean:   9232.90  61.21%  stddev:  41.80
user:  mean:  14759.40 100.00%  stddev:  30.62
       mean:   9100.10  61.66%  stddev:  41.75
sys:   mean:    324.00 100.00%  stddev:  39.51
       mean:    132.00  40.74%  stddev:  27.26
...
which is also paired with a reduction in peak memory usage of ~34%, from
0.95GB to 0.63GB, due to running into the low-mem die-limit in a more
efficient manner.

Thanks,
- Tom