chore: Remove ssa::value::Value to greatly reduce memory usage

[jfecher]

Description

Problem*

Working towards reducing memory usage in SSA for #3380

Summary*

This PR "inlines" the ssa::value::Value enum into ssa::value::ValueId to make the ValueId itself an enum and allow us to remove Value entirely. Doing this eliminates a large portion of memory usage as we no longer need to store values in separate maps at all. As an example, instruction_results: HashMap<InstructionId, Vec<Value>> alone takes up over 30% of the memory usage in #3380. In general, since ValueIds can be stored directly on Instructions, they are easier to free after each pass is finished so the total memory usage of the SSA representation grows less after each pass.

Additional Context

This PR is to better keep track of this change which currently is not close to completion.

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [Exceptional Case] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[jfecher]

We've had enough SSA changes since this PR was abandoned that I'm going to just restart the PR instead of trying to fix the conflicts.