Fix #162948: Vulkan: Math work around for SPIR-V compiler bug

Fix #162948.

This is up-stream issue in SpirV-Opt,
 see https://github.com/KhronosGroup/SPIRV-Tools/issues/6794,
 which is fixed but is not, as of now, in a build yet.

The affected ops are Divide and Snap, so
fold these cases before emitting shader code:

- Divide: fold `safe_divide(a, a)` to `1`.
- Snap: fold `floor(safe_divide(a, a)) * a` to `a`.

Pull Request: https://projects.blender.org/blender/blender/pulls/163039

source/blender/gpu/shaders/material/gpu_shader_material_vector_math.bsl.hh

@@ -41,6 +41,14 @@ void vector_math_divide(
   outVector = safe_divide(a, b);
 }
 
+/* `a / a` folded to 1 (see #162948). */
+[[node]]
+void vector_math_divide_self(
+    float3 a, float3 /*b*/, float3 /*c*/, float /*scale*/, float3 &outVector, float & /*outValue*/)
+{
+  outVector = float3(notEqual(a, float3(0.0f)));
+}
+
 [[node]]
 void vector_math_cross(
     float3 a, float3 b, float3 /*c*/, float /*scale*/, float3 &outVector, float & /*outValue*/)
@@ -110,6 +118,14 @@ void vector_math_snap(
   outVector = floor(safe_divide(a, b)) * b;
 }
 
+/* `floor(a / a) * a` folded to `a` (see #162948). */
+[[node]]
+void vector_math_snap_self(
+    float3 a, float3 /*b*/, float3 /*c*/, float /*scale*/, float3 &outVector, float & /*outValue*/)
+{
+  outVector = a;
+}
+
 [[node]]
 void vector_math_floor(
     float3 a, float3 /*b*/, float3 /*c*/, float /*scale*/, float3 &outVector, float & /*outValue*/)

source/blender/nodes/shader/nodes/node_shader_vector_math.cc

@@ -253,6 +253,18 @@ static int gpu_shader_vector_math(GPUMaterial *mat,
                                   GPUNodeStack *in,
                                   GPUNodeStack *out)
 {
+  /* Can't emit 'a / a' (see #162948). So we have to fold it. */
+  if (in[0].link != nullptr && in[0].link == in[1].link) {
+    switch (node->custom1) {
+      case NODE_VECTOR_MATH_DIVIDE:
+        return GPU_stack_link(mat, node, "vector_math_divide_self", in, out);
+      case NODE_VECTOR_MATH_SNAP:
+        return GPU_stack_link(mat, node, "vector_math_snap_self", in, out);
+      default:
+        break;
+    }
+  }
+
   const char *name = gpu_shader_get_name(node->custom1);
   if (name != nullptr) {
     return GPU_stack_link(mat, node, name, in, out);