[llvm] [RegAlloc] Scale the spill weight by target factor (PR #113675)

[Matt Arsenault via llvm-commits]

arsenm wrote:

I still think we should be making active effort to just make this the default. I believe this is another case where the target hook is just being used to paper over issues in other areas (I'm not objecting to this as an incremental step).

In particular in the previous revision of the patch, the AMDGPU behavior showed rematerialize was totally broken. I also think that something is wrong with whatever tablegen is doing to compute the default class weight. We have quite a lot of code scattered in the allocator and tablegen that were never written to account for subregisters. The defaults are wrong for the allocation priorities (which RISCV also does not look like it's trying to set). We also lack a splitting strategy to evict only a subregister when it would help, and spilling induces new liveness to dead lanes.

https://github.com/llvm/llvm-project/pull/113675