[mono] Improvement: avoid deep recursion in mono_ssa_rename_vars

[radekdoulik]

In #57141 I see that mono_ssa_rename_vars is entering deep recursion and was resulting in stack overflow on windows.

Consider to modify it to avoid recursion.

[SamMonoRT]

@radekdoulik -any more work remaining here ?

[radekdoulik]

It's done :-)