[llvm-dev] How does one match undef in tablegen?

[Jon Chesterfield via llvm-dev]

I would like to specialise build_vector for the case when one of the
operands is undefined. How do I describe this?

This is looking for an analog of specialisations like:
def : Pat <v2i32 (build_vector i32:$x, (i32 0)),...>;
but for an undefined, rather than zero, value.

I can work around my ignorance in performDAGCombine but would prefer to add
to the existing pattern matching.

Thanks,

Jon
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