[llvm] [LoopPeeling] Fix weights updating of peeled off branches (PR #70094)

[Aleksandr Popov via llvm-commits]

aleks-tmb wrote:

@MatzeB Hi, could you please take a look. Described issue manifested after your patch:
```
SHA: b30c9c937802a78ef986cb4219eba51148f76e6c
Author: Matthias Braun
Date: 2023-09-11 14:23:29 -0700 -0700
LoopUnrollRuntime: Add weights to all branches
```

Actually your patch added branch weights scaling. So imagine a loop with scaled latch weights ratio from 1:2 to 127:254 one. If we decide to peel 3 iterations of such loop, initial 1:2 ratio will be updated as:

```
0th peeled latch: 1:2
1st peeled latch: 1:1 
2nd peeled latch: 1:1 
```
And with initial 127:254:
```
0th peeled latch: 127:254
1st peeled latch: 127:127
2nd peeled latch: 127:1
```
So the hotness of the mail loop header changed from neutral 1:1 to 127:1, making it almost unreachable.

In my patch I tried to make it:

https://github.com/llvm/llvm-project/pull/70094