Print comments in MIR dumps aligned to the longest line

[oli-obk]

The comments in the following snippet should be aligned, otherwise they are hard to read, especially in larger blocks with lots of indentation changes.

        StorageLive(_1);                 // bb0[0]: scope 0 at $DIR/const_allocation.rs:8:5: 8:8
        StorageLive(_2);                 // bb0[1]: scope 0 at $DIR/const_allocation.rs:8:5: 8:8
        _2 = const {alloc0+0: &&[(std::option::Option<i32>, &[&str])]}; // bb0[2]: scope 0 at $DIR/const_allocation.rs:8:5: 8:8