Constants in MIR dumps are too verbose

[oli-obk]

As an example take

        _2 = const {alloc0+0: &&[(std::option::Option<i32>, &[&str])]}; // bb0[2]: scope 0 at $DIR/const_allocation.rs:8:5: 8:8
                                         // ty::Const
                                         // + ty: &&[(std::option::Option<i32>, &[&str])]
                                         // + val: Value(Scalar(alloc0+0))
                                         // mir::Constant
                                         // + span: $DIR/const_allocation.rs:8:5: 8:8
                                         // + literal: Const { ty: &&[(std::option::Option<i32>, &[&str])], val: Value(Scalar(alloc0+0)) }

We should trim some of that away and consider printing terminators in a multiline block format so multiple constants occurring in them are printed in a readable way