[SystemZ][z/OS] Enforce prefix-less registers in SystemZAsmParser for the HLASM dialect.

- Previously, https://reviews.llvm.org/D101308 removed prefixes from register while printing them out. This was especially needed for inline asm statements which used input/output operands.
- However, the backend SystemZAsmParser, accepts both prefixed registers and prefix-less registers as part of its implementation
- This patch aims to change that by ensuring that prefixed registers are only allowed for the ATT dialect.

Reviewed By: uweigand

Differential Revision: https://reviews.llvm.org/D101665