[IR] value name & AsmPrinter

[KihongHeo]

Hi,

I use Value::getName and Value::print to obtain the string representation of IR variables such as %x.
I have two questions:

• It seems that a value sometimes does not have a name. Could anyone explain when the name is available?
• When the name is NULL, I use print to obtain the string representation of an instruction and manipulate it on my own. But it seems print is extremely slow for large IR. Does anyone know some idea for optimizing this process?