Work towards enforcing to pass the shape to IrArray::Index constructors.

Instead of creating an IrArray::Index with multim initialized to nullptr,
create a std::vector<llvm::Value*> and once it is initialized, create an
IrArray::Index with shape information.
For scalar shapes, we still allow to call the IrArray::Index constructor with
passing just the index type.
PiperOrigin-RevId: 238399424