Support folding n-ary intrinsics in IRBuilderFolder/ConstantFolding #153743
Description
The motivation for this is #153069, where we would like to avoid regressions by being able to simplify unary and ternary intrinsics in IRBuilderFolder.
Currently we can only fold binary intrinsics. I presume we want to change the IRBuilderFolder interface from FoldBinaryIntrinsic -> FoldIntrinsic. I'm not sure what needs to change in ConstantFolder.cpp though. I see we have ConstantFoldBinaryIntrinsic, but we also seem to handle scalar intrinsics in ConstantFoldCall?
The only folding we really need for #153069 is propagating poison, i.e. intrinsicPropagatesPoison
Some relevant discussion in #150931 (comment)