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

[Pengcheng Wang via llvm-commits]

wangpc-pp wrote:

> E.g. there is the possibility that spilling one M8 register allows 8 M1 registers to avoid spills.

I don't think this is possible in current greedy RA (please correct me if my understanding is wrong). When spilling, we compare the weights of candidates one by one, because we can easily compare the live intervals of two registers. But when it comes to 8 registers, there are some problems: how can we compare the live intervals? how can compare the weight of one register with the weight of 8 registers (that means, how can we calculate one weight from 8 weights?), etc.

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