Improve stack optimization pass

• Fold stack_addr into load/store.
• Remove stack_{addr,load} with unused return value.
• Perform store to load forwarding when stack_addr is not used on a stack slot.
  • Single ebb store to load forwarding
  • Cross ebb store to load forwarding
  • Store to load forwarding with are multiple stores, but is always after others and before the load
  • Store to load forwarding with phi's
• Remove redundant stack_store. (no stack_load between current and next stack_store and no stack_addr before current stack_store)
• Fold stack_load into bitcast when stack_load is only used by that bitcast.

WIP implementation at https://github.com/bjorn3/rustc_codegen_cranelift/tree/opt_stack2reg
Edit: merged

No files, tests, or entry points are named. First verify the current stack optimization pass and whether the unchecked checklist items remain unresolved, since the issue notes that the WIP implementation was merged. Done would mean the remaining stack optimization cases are implemented and covered by the project's existing compiler tests.