i965/fs: Make emit_if_gen6 never fall back to emit_bool_to_cond_code.

i965/fs: Make emit_if_gen6 never fall back to emit_bool_to_cond_code.

Matt and I believe that Sandybridge actually uses 0xFFFFFFFF for a
"true" comparison result, similar to Ivybridge.  This matches the
internal documentation, and empirical results, but contradicts the PRM.

So, the comment is inaccurate, and we can actually just handle these
directly without ever needing to fall through to the condition code
path.

Also, the vec4 backend has always done it this way, and has apparently
been working fine.  This patch makes the FS backend match the vec4
backend's behavior.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>