[llvm] [SLP] Make getSameOpcode support interchangeable instructions. (PR #127450)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

I also ran into failed asserts since this change, so I pushed a revert.

The issue is reproducible with the following reduced input:
```c
void make_filters_from_proto(int *filter[][2], int bands) {
  int c, q, n;
  for (;; q++) {
    n = 0;
    for (; n < 7; n++) {
      int theta = (q * (n - 6) + (n >> 1) - 3) % bands;
      if (theta)
        c = theta;
      filter[q][n][0] = c;
    }
  }
}
```

Compiled e.g. like this:
```
$ clang -target x86_64-linux-gnu -c repro.c -O3
clang: ../lib/Transforms/Vectorize/SLPVectorizer.cpp:989: llvm::SmallVector<llvm::Value*> {anonymous}::BinOpSameOpcodeHelper::InterchangeableInfo::getOperand(llvm::Instruction*) const: Assertion `FromCIValue.isZero() && "Cannot convert the instruction."' failed.
```

The issue also reproduces by compiling ffmpeg with e.g. llvm-test-suite, according to the instructions at https://github.com/llvm/llvm-test-suite/blob/main/External/ffmpeg/README.md.

https://github.com/llvm/llvm-project/pull/127450