Make MIR basic blocks field public

[tmiasko]

This makes it possible to mutably borrow different fields of the MIR
body without resorting to methods like basic_blocks_local_decls_mut_and_var_debug_info.

To preserve validity of control flow graph caches in the presence of
modifications, a new struct BasicBlocks wraps together basic blocks
and control flow graph caches.

The BasicBlocks dereferences to IndexVec<BasicBlock, BasicBlockData>.
On the other hand a mutable access requires explicit as_mut() call.

[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[rust-highfive]

r? @petrochenkov

(rust-highfive has picked a reviewer for you, use r? to override)

[oli-obk]

r? @oli-obk

this is neat, definitely a massive improvement over the explosion of iteration methods ^^

[JakobDegen]

Yeah, agreed, this is a much better solution than what I did

[oli-obk]

@bors r+

[bors]

📌 Commit 802cb1c has been approved by oli-obk

[Dylan-DPC]

failed in rollup ci

@bors r-

[oli-obk]

@bors r+

[bors]

📌 Commit 17adfeb has been approved by oli-obk

[eddyb]

Neat! I guess the focus on basic_blocks: IndexVec<...> had obscured the deeper connection, before.

---

As a random aside, I believe some IRs would describe this subset of information as "the CFG" (though they might vary in whether that includes the BB contents, or just the terminators).

The only reason I bring this up is because body.cfg[bb] is quite compact and reads nicely IMO, but of course these are just subjective aesthetics - feel free to ignore this drive-by bikeshed comment.