[llvm] [SLP]Reuse the trimmed nodes in the perfectly matched gathers of the same user (PR #226582)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

This change broke compilation of ffmpeg for multiple architectures. Reduced testcase:
```c
float a;
float *b;
int c;
void d();
void e() {
  float f, g;
  for (; c; c++) {
    g += (a + b[c]) * (a + b[c]);
    f += (a - b[c]) * (a - b[c]);
  }
  d(f);
  d(g);
}
```
```console
$ clang -target x86_64-linux-gnu -c -O2 repro.c
Instruction does not dominate all uses!
  %8 = insertelement <2 x float> poison, float %sub, i64 0
  %7 = insertelement <2 x float> %8, float %add, i64 1
Instruction does not dominate all uses!
  %13 = insertelement <2 x float> poison, float %sub.1, i64 0
  %12 = insertelement <2 x float> %13, float %add.1, i64 1
in function e
fatal error: error in backend: Broken function found, compilation aborted!
```

https://github.com/llvm/llvm-project/pull/226582