Generate backend Drop flags for conditional moves

Objective

Connect Conditional BIR Drop results to backend cleanup with boolean Drop flags.

Why it was needed

A Conditional result means a value still exists on some runtime paths but was moved on others.

The compiler cannot decide at compile time whether Drop should run.

This patch adds a boolean flag, updates it after initialization or a move, and checks it during cleanup.

Main changes

• Creates the flag before initializing the local.
• Sets the flag to true after initialization.
• Clears the flag to false when the value is moved.
• Checks and clears the flag before the Drop call.

Test case

• drop-conditional-move.rs: Runs true and false paths for a one-branch move, a value that remains initialized, and a move on both branches.