[RISCV] Implement prefetch locality by NTLH

We add the MemOperand then backend will generate NTLH automatically.

```
__builtin_prefetch(ptr,  0 /* rw==read */, 0 /* locality */); => ntl.all + prefetch.r (ptr)
__builtin_prefetch(ptr,  0 /* rw==read */, 1 /* locality */); => ntl.pall + prefetch.r (ptr)
__builtin_prefetch(ptr,  0 /* rw==read */, 2 /* locality */); => ntl.p1 + prefetch.r (ptr)
__builtin_prefetch(ptr,  0 /* rw==read */, 3 /* locality */); => prefetch.r (ptr)
```

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D154691