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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

currently we only use 64bit registers, extend/truncate result if it is
not 64bits. This is awful. Going to use all the registers, for example,
AL, AX, EAX, RAX, but will not use AH (so reg alloc is simpler)

Note: made two places in api implementation as unimplemented as
the instruction/type chnaged.

(throw, new not working)

partially serialize compiled function into bootimage