[llvm] [SLP][REVEC] Make SLP support revectorization (-slp-revec) and add simple test. (PR #98269)

[Amy Huang via llvm-commits]

amykhuang wrote:

we hit an assert from this patch in chromium code

reduced c++ repro with `clang -cc1 -triple arm64-apple-macosx10.15.0 -O3 -vectorize-slp -emit-obj t.c -target-cpu apple-m1`

```
typedef __attribute__((neon_vector_type(8))) unsigned char a;
a b;
void c(long *g) {
  a d, e, f = __builtin_neon_vpadd_v(e, d, 16);
  long bitmap = ({ __builtin_neon_vget_lane_i64(b, 0); });
  g[0] = ~bitmap;
  bitmap = ({ __builtin_neon_vget_lane_i64(f, 0); });
  g[1] = ~bitmap;
}
```

https://github.com/llvm/llvm-project/pull/98269