Fix incorrect IO tile corner detection in nextpnr-aegis

The previous corner detection used `x != y` / `x == y` which only
excluded the (0,0) and (W-1,H-1) diagonal corners. The corners
(W-1,0) and (0,H-1) were incorrectly treated as valid IO tiles,
causing IO BELs, wires, and pips to be created at positions that
don't correspond to actual IO pads in the FPGA hardware.

This mismatch between the nextpnr model and the simulator/hardware
could cause the placer to assign IO cells to non-existent pad
positions, producing a bitstream that doesn't work on the device.

Fix by adding an is_corner() helper that correctly identifies all
four grid corners, and use it in init_wires, init_bels, and init_pips.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: 2 people

nextpnr-aegis/aegis.cc

@@ -180,6 +180,10 @@ struct AegisImpl : ViaductAPI {
     return (x == 0) || (x == (W - 1)) || (y == 0) || (y == (H - 1));
   }
 
+  bool is_corner(int x, int y) const {
+    return (x == 0 || x == W - 1) && (y == 0 || y == H - 1);
+  }
+
   PipId add_pip(Loc loc, WireId src, WireId dst, delay_t delay = 0.05) {
     IdStringList name =
         IdStringList::concat(ctx->getWireName(dst), ctx->getWireName(src));
@@ -238,7 +242,7 @@ struct AegisImpl : ViaductAPI {
             tw.track_w.push_back(ctx->addWire(h.xy_id(x, y, ctx->idf("W%d", t)),
                                               ctx->id("ROUTING"), x, y));
           }
-        } else if (x != y) {
+        } else if (!is_corner(x, y)) {
           // IO tile wires
           for (int z = 0; z < 2; z++) {
             tw.pad.push_back(ctx->addWire(h.xy_id(x, y, ctx->idf("PAD%d", z)),
@@ -269,7 +273,7 @@ struct AegisImpl : ViaductAPI {
     for (int y = 0; y < H; y++) {
       for (int x = 0; x < W; x++) {
         if (is_io(x, y)) {
-          if (x == y)
+          if (is_corner(x, y))
             continue;
           add_io_bels(x, y);
         } else {
@@ -320,7 +324,7 @@ struct AegisImpl : ViaductAPI {
     for (int y = 0; y < H; y++) {
       for (int x = 0; x < W; x++) {
         if (is_io(x, y)) {
-          if (x != y)
+          if (!is_corner(x, y))
             add_io_pips(x, y);
         } else {
           add_logic_pips(x, y);