agx: Ensure load_frag_coord has the right sizes

In case .x isn't read, it'll be null which has the wrong size and will fail
the validation added later in this series. We fix this by padding with sized
undefs (something that exists of defined size but undefined value) rather than
nothingness (of undefined size).

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22891>

src/asahi/compiler/agx_compile.c

@@ -636,7 +636,11 @@ static void
 agx_emit_load_frag_coord(agx_builder *b, agx_index dst,
                          nir_intrinsic_instr *instr)
 {
-   agx_index dests[4] = {agx_null()};
+   agx_index dests[4];
+
+   for (unsigned i = 0; i < ARRAY_SIZE(dests); ++i) {
+      dests[i] = agx_undef(AGX_SIZE_32);
+   }
 
    u_foreach_bit(i, nir_ssa_def_components_read(&instr->dest.ssa)) {
       agx_index fp32 = agx_temp(b->shader, AGX_SIZE_32);