[llvm] [VPlan] Compute scalar cost based on VPlan0 instead of legacy CM. (PR #196845)

[Luke Lau via llvm-commits]

https://github.com/lukel97 commented:

It looks like branches that never get executed end up losing their scalar cost divisor in this PR. E.g. with `opt -passes=loop-vectorize -mattr=+avx2` the scalar vplan cost goes from 4 -> 30, so the VF goes from VF 1 -> VF 8:

```llvm
define void @never_taken(ptr noalias %a, ptr noalias %b, i64 %n) {
entry:
  br label %loop

loop:
  %iv = phi i64 [ 0, %entry ], [ %iv.next, %latch ]
  %gep.a = getelementptr inbounds i32, ptr %a, i64 %iv
  %l = load i32, ptr %gep.a, align 4
  %c = icmp sgt i32 %l, 0
  br i1 %c, label %then, label %latch, !prof !0

then:
  %d = sdiv i32 1000, %l
  %gep.b = getelementptr inbounds i32, ptr %b, i64 %iv
  store i32 %d, ptr %gep.b, align 4
  br label %latch

latch:
  %iv.next = add nuw nsw i64 %iv, 1
  %ec = icmp eq i64 %iv.next, %n
  br i1 %ec, label %exit, label %loop

exit:
  ret void
}

!0 = !{!"branch_weights", i32 0, i32 1000}
```

I think this is related to the previous PRs that handled setExecutionFrequency?

https://github.com/llvm/llvm-project/pull/196845