i965: emit w/a for packed attribute formats in VS

i965: emit w/a for packed attribute formats in VS

Implements BGRA swizzle, sign recovery, and normalization
as required by ARB_vertex_type_10_10_10_2_rev.

V2: Ported to the new VS backend, since that's all that's left;
	fixed normalization.

V3: Moved fixups out of the GLSL-only path, so it works for FF/VP too.

V4 (Kayden): Rework ES3 normalization, don't heap allocate registers;
	tidy comments.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>