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

[Ankit Kumar Tiwari via llvm-commits]

ankit-cybertron wrote:

Confirmed #209190 already covers most of the  implementation, just added test for VECREDUCE_MUL.  Should be a much smaller review now 

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