Update for review comments

Ping

Ping

Update for review feedback.

Ping

Fix improperly updated testcase.

Incorporated build fixes for GCC. Waiting for buildbots to complete.

Fix nit

In D140952#4025370, @fhahn wrote:

Credit to czwarich for figuring out the algorithm to test for this.

Would be good to add a description.

Address review feedback.

Removing the FIXME in https://reviews.llvm.org/D141344

Update for comment

Rebase

Fix testcase.

Refactor and add more code comments. Update test comments.

Incorporate suggestion

I have not been able to construct a test for this manually, and didn't find any hits for it in the test-suite.

Update for feedback

Extract poison fixup logic to a helper and apply it in the short-circuit case without
going through the rest of the hoisting process.

Restart case iteration when we fold the switch condition.

Update for feedback

Reduce test case.

Update for comments

I don't have a test that exercises this, just following up on the FIXME comments.

Update for test comments and add tests.

Fix typo.

Update based on feedback.

Add negative tests. Enhance comments to explain the profitability heuristic.
Attempt to fix pre-existing correctness concerns WRT non-transferring predecessors.

Rebase

Rebase