[SystemZ][z/OS] Remove register prefixes when printing out the register.

- This patch is the first part in enforcing prefix-less registers for the HLASM dialect in z/OS
- This patch removes the "%[r|f|v]" prefix while printing registers
- To achieve this, the `AssemblerDialect` field of MAI was used
- There is also a bit of refactoring done to ensure code repetition is reduced.
- Currently the LLVM assembler for SystemZ/z/OS accepts both prefixed registers and prefix-less registers. A subsequent follow-up patch will restrict the SystemZAsmParser to only accept prefix-less registers.

Crediting @kianm as an author as well.

Reviewed By: uweigand, abhina.sreeskantharajan

Differential Revision: https://reviews.llvm.org/D101308