Remove workarounds to old GCC bugs we had encountered on IRIX

The check is bogus anyway. It was bogus when it was added. The bug is
not because of "GCC on IRIX", it's simply a GCC bug and was probably
tied to some GCC versions. It should have been reported and followed up.

I don't even remember what GCC versions we had on the IRIX machines
(plastkrakk, I can't remember the other two machine names). But I could
bet they were GCC 3.4 or 4.0.

Change-Id: I84ce4e1ad68bb0520b63c210f841e0c604dbd03a
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>
Fix corner-case counting of bits in QBitArray::count(bool)

This actually looks very wrong. First, it would try to read bits for
len == 0, which means it was actually reading the implicit NUL from
QByteArray (so valgrind would never catch the error).

Second, there was a corner case for testing the 8th bit (bit 7) in the
last byte. For len == 8 or 16 at the beginning of the last loop, it
would read bits[len / 8], which is again the implicit NUL from
QByteArray.

Compare to testBit (simplified):
    return d.constData()[1+(i>>3)] & (1 << (i & 7)) != 0;

Task-number: QTBUG-11625
Change-Id: Idb361163de596b629cab42f2367ddd09456c2a98
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Kurt Pattyn <pattyn.kurt@gmail.com>
Add a construction note on why QBitArray has a +1 everywhere

It took me several minutes reading the code to figure out why it was
there. It wasn't immediately obvious.

Change-Id: Ic36b3fd24ce84a1b08c73986d3b7ab8a5bfff133
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>
Improve QBitArray's construction performance a little

Ask for an uninitialized byte array, since we're about to memset(3) it
anyway. And don't overwrite the initial byte either.

Change-Id: I2caa2ef395ad5684416e6cd336c0444de7787b5d
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Fix last warning of qtbase with GCC.

qlistview.cpp:1836:6: error: unused parameter ‘step’ [-Werror=unused-parameter]

Change-Id: I8c2d16bd794204adafcf2dc049c04601506e7734
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>