[llvm] [SLSR] Adding a cost model considering register pressure (PR #213808)

[Igor Wodiany via llvm-commits]

https://github.com/IgWod commented:

Code looks good in general. I left few small comments. I'm still not convinced about using distance between IR instructions -- the ordering at this stage is somehow arbitrary. The code in your test could equally be written as:

```llvm
entry:
  %t1 = add i32 %b, %s
  %s2 = shl i32 %s, 1
  %t2 = add i32 %b, %s2
  call void @foo(i32 %t2)
  call void @foo(i32 %t1)
  call void @foo(i32 %b)
  call void @foo(i32 %b)
  call void @foo(i32 %b)
  br label %next

next:
  call void @use(i32 %s2)
  ret void
}
```

And this completely changes the distance. The heuristic only works because the code was generated and kept in a specific order before SLSR is run but in general it's not necessary.

https://github.com/llvm/llvm-project/pull/213808