radeonsi: zero init using the correct components count

This fixes the replay of the trace attached to
https://gitlab.freedesktop.org/mesa/mesa/-/issues/7530 which
would otherwise fail with an LLVM error:
  "All operands to PHI node must be the same type as the PHI node!"

Fixes: ae4379d81e ("ac/nir: export some undef as zero")
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19189>
(cherry picked from commit 434e74e658)

.pick_status.json

@@ -1957,7 +1957,7 @@
         "description": "radeonsi: zero init using the correct components count",
         "nominated": true,
         "nomination_type": 1,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": "ae4379d81e42dec4f93983dfa9f31cf30384789f"
     },

src/amd/llvm/ac_nir_to_llvm.c

@@ -5070,7 +5070,7 @@ static void visit_ssa_undef(struct ac_nir_context *ctx, const nir_ssa_undef_inst
    } else {
       LLVMValueRef zero = LLVMConstInt(type, 0, false);
       if (num_components > 1) {
-         zero = ac_build_gather_values_extended(&ctx->ac, &zero, 4, 0, false);
+         zero = ac_build_gather_values_extended(&ctx->ac, &zero, num_components, 0, false);
       }
       ctx->ssa_defs[instr->def.index] = zero;
    }