swr: [rasterizer jit] use signed integer representation for logic op

swr: [rasterizer jit] use signed integer representation for logic op

Instead of (incorrectly) biasing the snorm value to make it look like a
unorm, just use signed integer math.

This fixes arb_color_buffer_float-render GL_RGBA8_SNORM

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Tim Rowley <timothy.o.rowley@intel.com>