[llvm] [SelectionDAG] Fold VECREDUCE_MUL of a constant BUILD_VECTOR (PR #210126)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

> Hey @RKSimon, for VECREDUCE_MUL, if one element is 0 and another element is poison, should the fold result be 0 or poison?

poison - https://alive2.llvm.org/ce/z/c8LUSj

https://github.com/llvm/llvm-project/pull/210126