Properly treat FuelVMInstructions in the memcpyopt

[ironcev]

Description

This PR fixes the issue of FuelVMInstructions not being considered in the memcpyopt as storing into the destination. E.g., in this example:

script {
    entry fn main() -> u256 {
        local u256 x
        local u256 y
        local u256 z

        entry():
        v0 = get_local ptr u256, x
        v1 = get_local ptr u256, y
        v2 = get_local ptr u256, z

        mem_copy_val v2, v0
        wide add v0, v1 to v2
        v3 = load v2

        ret u256 v3
    }
}

after the memcpyopt the v3 = load v2 got turned into v3 = load v0 because the v2 is considered as not being written to.

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.