Rollup merge of #147858 - yotamofek:pr/mir/coroutine-layout-opt, r=cjgillot

Micro-optimization attempt in coroutine layout computation

In `compute_layout`, there were a bunch of collections (`IndexVec`s) that were being created by `push`ing in a loop, instead of a, hopefully, more performant usage of iterator combinators. [Second commit](rust-lang/rust@6f682c2) is just a small cleanup.

I'd love a perf run to see if this shows up in benchmarks.

Author: jhpratt