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

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

I'm also still hitting crashes after the reland, also in libavcodec (but I hit it in another source file first, so I've reduced that one):
```c
short *transform_4x4_luma_12_coeffs;
int transform_4x4_luma_12_i;
void transform_4x4_luma_12() {
  transform_4x4_luma_12_i = 0;
  for (; transform_4x4_luma_12_i < 4; transform_4x4_luma_12_i++) {
    int c2 = -transform_4x4_luma_12_coeffs[3];
    transform_4x4_luma_12_coeffs[2] = transform_4x4_luma_12_coeffs[1] + 7 >> 7;
    transform_4x4_luma_12_coeffs[0] = transform_4x4_luma_12_coeffs[1] =
        transform_4x4_luma_12_coeffs[3] + transform_4x4_luma_12_coeffs[1] >> 7;
    transform_4x4_luma_12_coeffs[3] = c2 >> 7;
    transform_4x4_luma_12_coeffs += 4;
  }
}
```

Compiled with:
```
$ clang -target x86_64-linux-gnu -c repro.c -O3
```

I can push a revert in a little while.

And before relanding, at this point it would probably be good to test building all of ffmpeg with the suggested patch.

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