Beta backport various arielb1 patches

[nikomatsakis]

Backport of the following PRs:

• refactor the handling of lvalue ops #41651
• try to fix lvalue ops for real #41828

cc @arielb1 @rust-lang/compiler

[rust-highfive]

r? @pnkfelix

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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!

[eddyb]

@bors r+

[bors]

📌 Commit 49138b4 has been approved by eddyb

[bors]

⌛ Testing commit 49138b4 with merge fb9ba8d...

[bors]

💔 Test failed - status-travis

[Mark-Simulacrum]

@bors retry

• Build just stopped

[alexcrichton]

@bors: r-

Looks like a legitimate error?

[00:48:58] test [run-pass] run-pass/issue-41696.rs has been running for over 60 seconds
...
No output has been received in the last 30m0s, this potentially indicates a stalled build or something wrong with the build itself.
Check the details on how to adjust your build configuration on: https://docs.travis-ci.com/user/common-build-problems/#Build-times-out-because-no-output-was-received
The build has been terminated

[nikomatsakis]

@arielb1 any idea why the backport [doesn't fix the problem for #41696](#42053 (comment)? is it maybe specific to what version of LLVM the beta is using or something?

[nikomatsakis]

Removed #41920

[nikomatsakis]

@bors r=eddyb

[bors]

📌 Commit 0afe53e has been approved by eddyb

[nikomatsakis]

@bors p=1

[bors]

⌛ Testing commit 0afe53e with merge 191cde0...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: eddyb
Pushing 191cde0 to beta...