[llvm] [AArch64] Add custom lowering for load <3 x i8>. (PR #78632)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

Actually, I guess the following is the shortest, at 2 instructions:

```
uint8x8_t load_3byte_insert_byte(char* a) {
  return vld1_lane_s8(a+2, vld1_dup_u16(a), 2);
}
```

https://github.com/llvm/llvm-project/pull/78632