sbrdsp: unroll sbr_autocorrelate_c

1410 cycles to 1148 on Arrandale/Win64.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
sbrdsp: unroll and use integer operations

This patch can be controversial, by assuming floats are IEEE-754 and
particular behaviour of the FPU will get in the way.
Timing on Arrandale and Win32 (thus, x87 FPU is used in the reference).

sbr_qmf_pre_shuffle_c: 115 to 76
sbr_neg_odd_64_c: 84 to 55
sbr_qmf_post_shuffle_c: 112 to 83

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>