Consider storing the instruction pointer in a local variable

It's currently stored in `ChunkCursor`, which is in the vm struct. We want to to be optimized to be stored in a register ideally.

Start by locating ChunkCursor and the vm struct, then trace how the instruction pointer is read and updated during VM execution. Check the existing execution path and tests, if present, before evaluating the local-variable approach. Done means preserving VM behavior while moving the instruction pointer out of ChunkCursor in the vm struct and enabling the intended optimization.

Written by the indexing model from the issue text.