[llvm] [Codegen] Add a separate stack ID for scalable predicates (PR #142390)

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

This is a stacked PR. Please see the final PR for context.

1. :point_right: https://github.com/llvm/llvm-project/pull/142390
2. https://github.com/llvm/llvm-project/pull/142391
3. https://github.com/llvm/llvm-project/pull/142392 

https://github.com/llvm/llvm-project/pull/142390