dropRedundantArguments was incorrectly indexing into forwardedOperands
using the block argument index directly. This crashes when the block has
produced operands (generated by the terminator, not forwarded from
predecessors) because forwardedOperands doesn't include them.
The fix checks isOperandProduced() to skip produced arguments and uses
SuccessorOperands::operator[] which handles the offset correctly.
4ac6431755