riscv64: Add comment into Riscv64Assembler::Add

In case of 'add rd, Zero, Zero' we could use
conversion into 'c.li rd, 0' but for some reason
clang does not support it so it could not be
tested.

Test: None
Bug: 328561342
Change-Id: I6efab426bf72330e3df2d6122bc83a3499ac4c9e
1 file changed