Backport #34141 to beta

[alexcrichton]

Didn't apply cleanly, but @eddyb mentioned it was ok to just trash all the trans updates, so this just changes old trans.

Beta backport of #34141

[rust-highfive]

r? @nikomatsakis

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[rust-highfive]

Warning

• Pull requests are usually filed against the master branch for this repo, but this one is against beta. Please double check that you specified the right target!

[alexcrichton]

To clarify, when backporting the commit from #34141, there were only conflicts in src/librustc_trans/mir/block.rs. I threw away all of those conflicts in favor of the code that was originally there. All other changes in other files applied cleanly, however, and are included here. I've also run make check locally and everything checks out

[eddyb]

For posterity's sake, the reason I said it's okay to remove those changes is that src/librustc_trans/mir/block.rs is actually correct on beta.
Half of the changes in that file from #34141 are reverting a (less correct) fix which didn't get into beta and the other half are for Pair handling, is which is also not in beta.