[llvm] [VPlan] Detect and create partial reductions in VPlan. (NFCI) (PR #167851)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

This change causes failed assertions, when compiling ffmpeg for i686 windows.
Repro:
```c

int a, e;
struct {
  char b
} *c;
char *d;
void f() {
  int g;
  for (int h = 0; h < a; h++)
    g += d[c[h].b] + c[h].b;
  e = g;
}
```
```
$ clang -target i686-windows-gnu -c -O2 repro.c
clang: ../include/llvm/Support/Casting.h:572: decltype(auto) llvm::cast(From*) [with To = VPWidenCastRecipe; From = VPValue]: Assertion `isa<To>(Val) && "cast<Ty>() argument of incompatible type!"' failed.
```


https://github.com/llvm/llvm-project/pull/167851