Use qregister(u)int in qsimd.cpp x86 code

This way, the same codebase works for x86, x86-64 LP64 and x86-64 ILP32
(a.k.a. x32).

Task-number: QTBUG-35463
Change-Id: Iffcecc6a4c03267fb64e287c2d6226d078d7d115
Reviewed-by: hjk <hjk121@nokiamail.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Update the macro that MSVC 2013 defines for AVX code generation

http://msdn.microsoft.com/en-us/library/b0084kay(v=vs.120).aspx says:
  __AVX__       Defined when /arch:AVX is specified.

Now we know what flag it is, we don't need to use our _M_AVX flag
anymore. We're also now assuming that Microsoft will follow the same
pattern for AVX2 (i.e., __AVX2__), so this commit also removes the
check for _M_AVX2.

The other defines that were defined alongside AVX2 are removed because
they have no use currently in Qt.

Change-Id: I64a026b2206dbd0d2dffa7c803bee969c9b94a94
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>