rvc: allow decode C.MV to ADDI with useAddiForMv (#3179)

This commit changes how compressed move instructions are decoded.
It adds an useAddiForMv argument to RVCDecoder to allow the user to
specify how C.MV is decoded.

From RISC-V spec, C.MV expands to a different instruction than the
canonical MV pseudoinstruction, which instead uses ADDI.
Implementations that handle MV specially, e.g. using register-renaming
hardware, may find it more convenient to expand C.MV to MV instead of
ADD, at slight additional hardware cost.

We add a configuration option for decoding C.MV to ADDI.