[RISCV] Precommit test for D124509

Test case to show the wrong code gen for `int16` -> `__fp16` conversion,
clang just emit a load and store without did conversion in the case,

and another case used for demonstrate the code gen change of `__fp16`
-> `int16`.

Reviewed By: khchen

Differential Revision: https://reviews.llvm.org/D124510