[opus] ARM NEON optimization -- celt_fir()

[Linfeng Zhang]

Hi all,

This is Linfeng Zhang from Google. I'll work on ARM NEON optimization in the
next few months.

I'm submitting 2 patches in the following couple of emails, which have the new
created celt_fir_neon().

I revised celt_fir_c() to not pass in argument "mem" in Patch 1. If there are
concerns to this change, please let me know.

Many thanks to your comments.

Linfeng Zhang