[RISCV] Simplify mergeCondAndCode in RISCVCompressInstEmitter.cpp. NFC

[RISCV] Simplify mergeCondAndCode in RISCVCompressInstEmitter.cpp. NFC

Instead forming a std::string and returning it to pass into another
raw_ostream, just pass the raw_ostream as a parameter.

Take StringRef as arguments instead raw_string_ostream references
making the caller responsible for converting to strings. Use
StringRef operations instead of std::string::substr.a