spirv-opt: fix copy-propagate-arrays index opti on structs.

As per SPIR-V spec:
OpAccessChain indices must be OpConstant when indexing into a structure.

This optimization tried to remove load cascade. But in some scenario
failed:

```c
cbuffer MyStruct {
    uint my_field;
};

uint main(uint index) {
    const uint my_array[1] = { my_field };
    return my_array[index]
}
```

This is valid as the struct is indexed with a constant index, and then
the array is indexed using a dynamic index.
The optimization would consider the local array to be useless and
generated a load directly into the struct.

Fixes KhronosGroup#4887

Signed-off-by: Nathan Gauër <brioche@google.com>

source/opt/copy_prop_arrays.cpp

@@ -506,6 +506,7 @@ bool CopyPropagateArrays::CanUpdateUses(Instruction* original_ptr_inst,
 
   return def_use_mgr->WhileEachUse(original_ptr_inst, [this, type_mgr,
                                                        const_mgr,
+                                                       def_use_mgr,
                                                        type](Instruction* use,
                                                              uint32_t) {
     if (IsDebugDeclareOrValue(use)) return true;
@@ -524,6 +525,11 @@ bool CopyPropagateArrays::CanUpdateUses(Instruction* original_ptr_inst,
         analysis::Pointer* pointer_type = type->AsPointer();
         const analysis::Type* pointee_type = pointer_type->pointee_type();
 
+        // Indexes must be an OpConstant when indexing in a struct. Cannot optimize further.
+        if (pointee_type->kind() == analysis::Type::kStruct) {
+          return false;
+        }
+
         std::vector<uint32_t> access_chain;
         for (uint32_t i = 1; i < use->NumInOperands(); ++i) {
           const analysis::Constant* index_const =
@@ -542,7 +548,7 @@ bool CopyPropagateArrays::CanUpdateUses(Instruction* original_ptr_inst,
         analysis::Pointer pointerTy(new_pointee_type,
                                     pointer_type->storage_class());
         uint32_t new_pointer_type_id =
-            context()->get_type_mgr()->GetTypeInstruction(&pointerTy);
+            type_mgr->GetTypeInstruction(&pointerTy);
         if (new_pointer_type_id == 0) {
           return false;
         }