llvm-reduce: Reduce individual operands of named metadata

llvm-reduce: Reduce individual operands of named metadata

The current reduction tries all or nothing elimination of named
metadata. I noticed in one case where one of the module flags was
necessary, but it left the rest. Reduce the individual operands of
named metadata nodes that are known to behave like lists. Be
conservative since some named metadata may have more specific verifier
requirements for the operands.