[RISCV] Fix RISCVAsmParser::ParseRegister and add tests

RISCVAsmParser::ParseRegister is called from AsmParser::parseRegisterOrNumber,
which in turn is called when processing CFI directives. The RISC-V
implementation wasn't setting RegNo, and so was incorrect. This patch address
that and adds cfi directive tests that demonstrate the fix. A follow-up patch
will factor out the register parsing logic shared between ParseRegister and
parseRegister.

llvm-svn: 356329