lvp: fix load_helper_invocation not being lowered

lvp: fix load_helper_invocation not being lowered

nir_lower_is_helper_invocation should be after nir_lower_system_values
to handle possible nir_intrinsic_is_helper_invocation which may be
produced by nir_lower_system_values.

Happens with SPIR-V 1.6 for which gl_HelperInvocation is translated into
"BuiltIn HelperInvocation" + "Volatile", which nir_lower_system_values
translates into is_helper_invocation.

Signed-off-by: Danylo Piliaiev <dpiliaiev@igalia.com>
Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19677>