[Clang][RISCV] Avoid generating MaskedPrototype if the intrinsic does not have...

[Clang][RISCV] Avoid generating MaskedPrototype if the intrinsic does not have a masked version. NFC

The function should not be called if an intrinsic does not have a masked version.

Signed-off by: eop Chen <eop.chen@sifive.com>