Re: speed up verifying UTF-8

My v16 experimental patches were a bit messy, so I've organized an
experimental series that applies cumulatively, to try to trace the effects
of various things.

v17-0001 is the same as v14. 0002 is a stripped-down implementation of
Amit's chunk idea for multibyte, and it's pretty good on x86. On Power8,
not so much. 0003 and 0004 are shot-in-the-dark guesses to improve it on
Power8, with some success, but end up making x86 weirdly slow, so I'm
afraid that could happen on other platforms as well.

v14 still looks like the safe bet for now. It also has the advantage of
using the same function both in and out of the fastpath, which will come in
handy when moving it to src/port as the fallback for SSE.

Power8, gcc 4.8:

HEAD:
 chinese | mixed | ascii | mixed16 | mixed8
---------+-------+-------+---------+--------
    2944 |  1523 |   871 |    1473 |   1509

v17-0001:
 chinese | mixed | ascii | mixed16 | mixed8
---------+-------+-------+---------+--------
     888 |   607 |   179 |     777 |   1328

v17-0002:
 chinese | mixed | ascii | mixed16 | mixed8
---------+-------+-------+---------+--------
    1017 |   718 |   156 |    1213 |   2138

v17-0003:
 chinese | mixed | ascii | mixed16 | mixed8
---------+-------+-------+---------+--------
    1205 |   662 |   180 |     767 |   1256

v17-0004:
 chinese | mixed | ascii | mixed16 | mixed8
---------+-------+-------+---------+--------
    1085 |   660 |   224 |     868 |   1369


Macbook x86, clang 12:

HEAD:
 chinese | mixed | ascii | mixed16 | mixed8
---------+-------+-------+---------+--------
     974 |   691 |   370 |     456 |    526

v17-0001:
 chinese | mixed | ascii | mixed16 | mixed8
---------+-------+-------+---------+--------
     674 |   346 |    78 |     309 |    504

v17-0002:
 chinese | mixed | ascii | mixed16 | mixed8
---------+-------+-------+---------+--------
     516 |   324 |    78 |     331 |    544

v17-0003:
 chinese | mixed | ascii | mixed16 | mixed8
---------+-------+-------+---------+--------
     621 |   537 |   323 |     413 |    602

v17-0004:
 chinese | mixed | ascii | mixed16 | mixed8
---------+-------+-------+---------+--------
     576 |   439 |   154 |     557 |    915

--
John Naylor
EDB: http://www.enterprisedb.com