Update silk_biquad_alt()

Split to silk_biquad_alt_stride1() and silk_biquad_alt_stride2(),
so that it can be optimized more efficiently when stride is 2.

This change in C code is bit exact with the origin.

Change-Id: Idaefe670397016ace2a489e3435ac61b7dbe79d5

Signed-off-by: Jean-Marc Valin <jmvalin@jmvalin.ca>