ac: fix ac_build_is_helper_invocation when postponed_kill is null

If there was no demote() in the shader, ac_build_is_helper_invocation
behaves exactly the same as ac_build_load_helper_invocation, i.e.
the helper lanes are the same as they were at the beginning of the shader.

Fixes: de57ea2a3d ("amd/llvm: implement nir_intrinsic_demote(_if) and nir_intrinsic_is_helper_invocation")
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4301>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4301>

src/amd/llvm/ac_llvm_build.c

@@ -4712,6 +4712,9 @@ ac_build_load_helper_invocation(struct ac_llvm_context *ctx)
 LLVMValueRef
 ac_build_is_helper_invocation(struct ac_llvm_context *ctx)
 {
+	if (!ctx->postponed_kill)
+		return ac_build_load_helper_invocation(ctx);
+
 	/* !(exact && postponed) */
 	LLVMValueRef exact = ac_build_intrinsic(ctx, "llvm.amdgcn.ps.live",
 						ctx->i1, NULL, 0,