[RISCV] Fix int16 -> __fp16 conversion code gen

clang emit wrong code sequence for `int16`(`short`) to `__fp16` conversion,
and that should fix the code gen directly is the right way I think,
but I found there is a FIXME comment in clang/Basic/TargetInfo.h say
that's should be removed in future so I think just let swich to using
generic LLVM IR rather than llvm.convert.to.fp16 intrinsics code gen
path is enough.

```
  /// Check whether llvm intrinsics such as llvm.convert.to.fp16 should be used
  /// to convert to and from __fp16.
  /// FIXME: This function should be removed once all targets stop using the
  /// conversion intrinsics.
  virtual bool useFP16ConversionIntrinsics() const {
    return true;
  }
```

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D124509