[RISCV] Fix RISCVAsmParser::ParseRegister and add tests

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@356329 91177308-0d34-0410-b5e6-96231b3b80d8