How to deal with UNDEF SDNode?

Hi All,

The comment of UNDEF in ISDOpcodes.h [1] is pretty simple,

/// UNDEF - An undefined node.


The name make me think of C undefined behavior, which is … scary. But I also see there are
APIs like isBuildVectorAllOnes, isBuildVectorAllZeros, … etc, seems UNDEF just can be any

value and we don’t care about what it is. The most case I see UNDEF is in BUULD_VECTOR,
VECTOR_SHUFFLE, … etc. I would like to know usually how we deal with those UNDEF nodes?