freedreno/ir3: lower extract_byte/word

freedreno/ir3: lower extract_byte/word

The following commits broke things by starting to feed us unhandled
extract_u16/extract_u8 opcodes:

commit 905ff861982450831a56d112036f68a751337441
Author:     Matt Turner <mattst88@gmail.com>
AuthorDate: Wed Feb 3 14:28:31 2016 -0800
Commit:     Matt Turner <mattst88@gmail.com>
CommitDate: Fri Mar 4 11:52:34 2016 -0800

    nir: Recognize open-coded extract_u16.

commit 76289fbfa84a06ef4db8ad44ea0eb88ad0be8d5c
Author:     Matt Turner <mattst88@gmail.com>
AuthorDate: Thu Jan 21 09:09:48 2016 -0800
Commit:     Matt Turner <mattst88@gmail.com>
CommitDate: Fri Mar 4 11:52:34 2016 -0800

    nir: Recognize open-coded extract_u8.

Signed-off-by: Rob Clark <robclark@freedesktop.org>