Auto merge of #111813 - scottmcm:pretty-mir, r=cjgillot

MIR: opt-in normalization of `BasicBlock` and `Local` numbering

This doesn't matter at all for actual codegen, but after spending some time reading pre-codegen MIR, I was wishing I didn't have to jump around so much in reading post-inlining code.

So this add two passes that are off by default for every mir level, but can be enabled (`-Zmir-enable-passes=+ReorderBasicBlocks,+ReorderLocals`) for humans.

Author: bors