[spirv] Avoid NaN when a conversion to f16 is emitted

Fixes black squares in Fire Emblem: Three Houses.
And fixes vertex explosions in The Legend of Zelda: Breath of the Wild shrine areas.
These were caused by NaN values from overflowed conversions to float16.

src/shader_recompiler/backend/spirv/emit_spirv_convert.cpp

@@ -1,3 +1,5 @@
+// SPDX-FileCopyrightText: 2025 Eden Emulator Project
+// SPDX-License-Identifier: GPL-3.0-or-later
 // SPDX-FileCopyrightText: Copyright 2021 yuzu Emulator Project
 // SPDX-License-Identifier: GPL-2.0-or-later
 
@@ -148,7 +150,10 @@ Id EmitConvertU32U64(EmitContext& ctx, Id value) {
 }
 
 Id EmitConvertF16F32(EmitContext& ctx, Id value) {
-    return ctx.OpFConvert(ctx.F16[1], value);
+    const auto result = ctx.OpFConvert(ctx.F16[1], value);
+    const auto isOverflowing = ctx.OpIsNan(ctx.U1, result);
+    return ctx.OpSelect(ctx.F16[1], isOverflowing, ctx.Constant(ctx.F16[1], 0), result);
+    //return ctx.OpFConvert(ctx.F16[1], value);
 }
 
 Id EmitConvertF32F16(EmitContext& ctx, Id value) {