vc4: enable lower_isign for VC4

Since 1e7d82c881 ("nir/algebraic: always lower idiv
to shifts if bitops are allowed") idiv is lowered and
generates a isign operation.

VC4 HW doesn't support isign and lower_isign wasn't enabled.
Enabling it fixes the regressions caused by this new
optimization on piglit tests shaders/glsl-fs-loop-nested.

Fixes: 1e7d82c881 ("nir/algebraic: always lower idiv to shifts if bitops are allowed")
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7089>

src/gallium/drivers/vc4/vc4_program.c

@@ -2193,6 +2193,7 @@ static const nir_shader_compiler_options nir_options = {
         .lower_to_scalar = true,
         .lower_umax = true,
         .lower_umin = true,
+        .lower_isign = true,
         .max_unroll_iterations = 32,
 };