[llvm] [SLP]Remove operands upon marking instruction for deletion. (PR #97409)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

This change has caused failed asserts. Strangely enough, the failed asserts aren't entirely deterministic though. Reduced testcase:

```c
int b;
float c;
int e;
void f(float p1[]) {
  float *j = &b;
  for (e = 0; e < 2;) {
    float a;
    float d;
    {
      a += p1[e] * 0;
      d += e * j[e];
    }
    e = a;
    c = d;
  }
}
```

To reproduce:
```
$ clang -target i686-linux-gnu -c repro.c -O2 -g
clang: ../lib/IR/Instruction.cpp:153: void llvm::Instruction::insertBefore(llvm:
:BasicBlock&, llvm::iplist_impl<llvm::simple_ilist<llvm::Instruction, llvm::ilis
t_iterator_bits<true>, llvm::ilist_parent<llvm::BasicBlock> >, llvm::SymbolTable
ListTraits<llvm::Instruction, llvm::ilist_iterator_bits<true>, llvm::ilist_paren
t<llvm::BasicBlock> > >::iterator): Assertion `!isa<PHINode>(this) && "Inserting
 PHI after debug-records!"' failed.
```

Please have a look, and consider reverting if fixing takes a long time.

https://github.com/llvm/llvm-project/pull/97409