QHostAddress: Improve code generation

QHostAddress: Improve code generation

Mostly related to IPv6, because Q_IPV6ADDR is an array of char, so the
compilers were generating byte access to each value. Instead, force
access as 32- and 64-bit in most places that make sense (64-bit access
decays to 32-bit on 32-bit machines). In one isLoopback(), this is now a
128-bit access for best improvement.

Some smaller improvements relating to SpecialAddress by combining the
three IPv4 special addresses.

Change-Id: I7de033f80b0e4431b7f1ffff13f932b1cd7b5d21
Reviewed-by: Richard J. Moore <rich@kde.org>