llvmpipe: do transpose/untwiddle after conversion for 8bit formats

llvmpipe: do transpose/untwiddle after conversion for 8bit formats

Generally we should do tranpose after conversion, if the format has less than
32 bits per channel (if it has 32 bits, conversion is going to be a no-op
anyway...). This is obviously because there's less vectors to deal with.
Though the advantage for 16 bit formats isn't that big, and in fact with AVX
there isn't really any (as the 32bit unpacks can be done with 256bit, but
the smaller ones cannot, although that would change again with proper AVX2
support).
Only makes sense for 2d and not 1d cases. And to keep things easy, only handle
1,2 and 4 channels (rgbx is just fine).
For rgba unorm8 format the backend conversion sums up to these instruction
totals (not counting the movs for SSE2 due to 2-op syntax - generally every 2
unpacks need an additional mov).
                     SSE2                    AVX
transpose:           32 unpack               16 unpack
untwiddle:           0                       8 (128bit low/high permutes)
convert:             16 mul + 16 cvt         8 mul + 8 cvt
32->8bit:            12 pack                 8 (128bit extract) + 12 pack

When doing transpose/untwiddle afterwards we get:
convert:             16 mul + 16 cvt         8 mul + 8 cvt
32->8bit:            12 pack                 8 (128bit extract) + 12 pack
transpose/untwiddle  12 unpack               12 unpack

So for SSE2, this drops 20 unpacks (total instruction count 76->56)
whereas for AVX it replaces the 16 256bit unpacks with 8 128bit ones
and drops the 8 lo/hi permutes (in total 60->48). (Albeit to be fair,
the permutes could be dropped even when doing the transpose first,
they are extremely pointless but we'd need to be able to tell
lp_build_conv to reorder the vectors, for AVX2 we're going to need to
be able to tell lp_build_conv about ordering in any case.)

(With different ordering going into conversion, it would be possible
to do 4 unpacks + 4 pshufbs instead of 12 unpacks, but that might not
be better, and not all cpus can do it. Proper AVX2 support should eliminate
the 8 128bit extracts, reduce these 12 packs to 6 and the 12 unpacks to 2
pshufb + 2 permq ideally (+ 2 final 128bit extracts).)

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>