[RISCV] Support scalar/fix-length vector NTLH intrinsic with different domain

This commit implements the two NTLH intrinsic functions.

```
type __riscv_ntl_load (type *ptr, int domain);
void __riscv_ntl_store (type *ptr, type val, int domain);

```

```
enum {
  __RISCV_NTLH_INNERMOST_PRIVATE = 2,
  __RISCV_NTLH_ALL_PRIVATE,
  __RISCV_NTLH_INNERMOST_SHARED,
  __RISCV_NTLH_ALL
};
```

We encode the non-temporal domain into MachineMemOperand flags.

1. Create the RISC-V built-in function with custom semantic checking.
2. Assume the domain argument is a compile time constant,
and make it as LLVM IR metadata (nontemp_node).
3. Encode domain value as two bits MachineMemOperand TargetMMOflag.
4. According to MachineMemOperand TargetMMOflag, select corrsponding ntlh instruction.

Currently, it supports scalar type and fixed-length vector type.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D143364