freedreno/ir3: fix load_front_face conversion

freedreno/ir3: fix load_front_face conversion

The comments are correct - we get -1 and 0. However by adding 1, we
convert this into 0,1. This mostly works for conditionals, but when
negated, this will yield the wrong result. Instead just negate the
values (as they are backwards -- -1 means back instead of front).

Fixes tests/shaders/glsl-fs-frontfacing-not.shader_test and
dEQP-GLES3.functional.shaders.builtin_variable.frontfacing on A530.

The latter also tested on A306 by Rob Clark.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>