Changes

[RISC] Replace dyn_casts that are only checked by an assert with a cast. 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

[RISCV] Few more minor cleanups to RISCVCompressInstEmitter. NFC

-Use StringRef instead of std::string.
-Const correct a parameter.
-Don't call StringRef::data() before printing. Just pass the StringRef.

[RISCV] Remove an extra map lookup from RISCVCompressInstEmitter. NFC

When we looked up the map to see if the entry already existed,
this created the new entry for us. So save a reference to it so
we can use it to update the entry instead of looking it up again.

Also remove unnecessary StringRef constructors around string
literals on calls to this function.