[Opt] Fix i64 indexes for matrices

[hughperkins]

Issue: #8640

#8640

Brief Summary

Using i64 index for matrix fires an assert is_primitive( PrimitiveTypeID::i32)

copilot:summary

Walkthrough

Using i64 index for matrix fires an assert:

[E 04/27/25 16:53:32.590 1740211] [type_check.cpp:visit@166] Assertion failure: stmt->offset->ret_type.get_element_type()->is_primitive( PrimitiveTypeID::i32)

The assert is because the verifier only allows matrix indices to be i32:

    TI_ASSERT(stmt->offset->ret_type.get_element_type()->is_primitive(
                  PrimitiveTypeID::i32));

Broadening this to allow i64 fixes the issue:

    TI_ASSERT(stmt->offset->ret_type.get_element_type()->is_primitive(
                  PrimitiveTypeID::i32) ||
              stmt->offset->ret_type.get_element_type()->is_primitive(
                  PrimitiveTypeID::i64));

copilot:walkthrough

[hughperkins]

Hmmm, I wonder if i64 will overflow our matrix indexing 🤔

[hughperkins]

Actually, this is probably a bad idea 🤔 Will close it for now.