[llvm] [StackSlotColoring] Fix issue where colors for a StackID are dropped (PR #138140)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> So, to get the stack slot liveness information, the register allocator needs to run and create stack slots. Since the register allocator makes the stack slots, I can't edit them to have the `noalloc` StackID. If I run the register allocator separately to generate the MIR, there's no stack slot liveness information, so the `stack-slot-coloring` pass is a nop.

What if you have pre-existing frame indexes, and then run through the allocator? 

https://github.com/llvm/llvm-project/pull/138140