[RISCV] Remove unneeded StringRef to std::string conversions in RISCVCompressInstEmitter. NFC

Stop concatenating std::string before streaming into a raw_ostream.
Just stream the pieces.

Remove some new lines from asserts. Remove std::string concatenation
from an assert. assert strings aren't really evaluated like this at
runtime. An assertion failure will just print exactly what's between
the parentheses in the source.