[llvm] Add a threshold to RegStackify to avoid register spills at runtime (PR #97283)

[Heejin Ahn via llvm-commits]

aheejin wrote:

So in which case are deep stacks bad for performance and why? Reading @eqrion and @jakobkummerow's comments, it doesn't sound like deep stacks are necessarily bad. @jakobkummerow noted that it can be bad to have too many short-lived locals but it's not the case here right?

https://github.com/llvm/llvm-project/pull/97283