Most addresses in SystemZ instructions take two registers,
an index register and a base register. However, either of
those can be omitted. If there is just a single register,
this usually is taken as the base register - however, there
are certain rare cases where you specifically want to use
an index register but no base register. This is currently
not handled consistently by the assembler / disassembler.
Fix this by
- always emitting a dummy 0 as base register for index-
only addresses
- correctly handle dummy 0 as indicating no base register
when parsing an address
This is compatible with current GNU binutils behavior.
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