[opus] Alleged bug in Silk codec

[Timothy B. Terriberry]

KOEN VOS wrote:
> current implementation is faster on a 32 or 16 bit processor.  If you
> would find the opposite to be true (ie that a 64 bit implementation is
> faster on, say, a 32 bit ARM CPU) then perhaps we should reconsider.

Doesn't ARMv6 have a dual signed 16x16->32 multiply with a 64-bit 
accumulator (SMLALD)? Even v5E should have a single 16x16->32 with a 
64-bit accumulator (SMLALBB). I would think a 64-bit version could be 
made pretty fast on 32-bit ARM, without even resorting to SIMD.