LoopPeel currently considers PHI nodes that become loop invariants
through peeling. However, in some cases, peeling transforms PHI nodes
into induction variables (IVs), potentially enabling further
optimizations such as loop vectorization. For example:
```c
// TSVC s292
int im = N-1;
for (int i=0; i<N; i++) {
a[i] = b[i] + b[im];
im = i;
}
```
In this case, peeling one iteration converts `im` into an IV, allowing
it to be handled by the loop vectorizer.
This patch adds a new feature to peel loops when to convert PHIs into
IVs. At the moment this feature is disabled by default.
Enabling it allows to vectorize the above example. I have measured on
neoverse-v2 and observed a speedup of more than 60% (options: `-O3
-ffast-math -mcpu=neoverse-v2 -mllvm -enable-peeling-for-iv`).
This PR is taken over from #94900
Related #81851
2330fd2f73