[llvm] [GlobalISel] Encode root operand indices in match opcodes (PR #210562)

[David Zbarsky via llvm-commits]

dzbarsky wrote:

> How was the limit of 8 chosen?

I ran a sweep: 
<img width="578" height="307" alt="image" src="https://github.com/user-attachments/assets/4e86f068-f205-4ce3-b076-c21ce138ee8d" />

7 is probably a reasonable choice as well but 8 is a more beautiful number :) LMK if you'd prefer a different one

https://github.com/llvm/llvm-project/pull/210562