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should be spilled. Add a simple mechanic to stop possible infinite loop
in regalloc.

This is mostly in service of implementing TagRef64, which can
now be initialized by a bundle (but is as of yet not properly
implemented).

with RegGroup::get_from_value

(cannot load this test as dylib, it would involve PIC code on linux)

info

when we add interefence between %a and %edi, we also add %a with %rdi
because we cannot assign rdi to a

1. see vm/vm_options.rs for usage and default values
2. added mu_fastimpl_new_with_opts under vm/api/api_impl/muvm.rs

internally int1 is int8

1. compiler knows all the registers
2. but only 64bits register is a color (for reg alloc)
3. backend records the length of GPR for each operand during instruction
selection
4. after reg alloc, when replacing temp with a color, find corresponding
GPR for the length recorded before