[llvm] [llvm][SystemZ] Fix parsing of `.cfi_undefined` with percent-less registers. (PR #107032)

[Alex Rønne Petersen via llvm-commits]

alexrp wrote:

Ok, this iteration of the patch rejects percent-less registers as instruction operands and in addresses as before. It only allows omitting the percent token in the overridden `bool SystemZAsmParser::parseRegister(MCRegister &Reg, SMLoc &StartLoc, SMLoc &EndLoc)` and `ParseStatus SystemZAsmParser::tryParseRegister(MCRegister &Reg, SMLoc &StartLoc, SMLoc &EndLoc)` methods, matching the x86 backend.

https://github.com/llvm/llvm-project/pull/107032