Remove vp.reverse

Currently, vp.reverse is only used for reverse accesses. Taking a reverse load as an example:
```llvm
vp.reverse(
vp.load(vec_end_ptr(addr, evl), true /* mask */, evl),
true /* mask */,
evl
)
```

Its semantics are equivalent to:
```llvm
vector.reverse(
vector.splice(
poison,
vp.load(vec_end_ptr(addr, evl), true /* mask */, evl),
evl
)
)
```

With this approach, we no longer need the vp.reverse intrinsic. Moreover, after the reverse operation is simplified away, optimizeMaskToEVL can still transform it into an reverse load/store with EVL tail folding.