Update commit message, didn't realize that arc diff overwrote it

Fix commit message typo

Address comments, redo description

@qcolombet Apologies if I'm pinging the wrong person here, I'm looking for some advice on some register coalescer behaviour, let me know if this question is better directed towards someone else.
On RISC-V we're seeing some unexpected extra COPYs being left around with these $x0 registers, which is both a reserved and constant physical register.

Add another test that should be fixed

Simplify test

In D142551#4083916, @nikic wrote:

Huh, this made compile-time impact even worse: https://llvm-compile-time-tracker.com/compare.php?from=051bcd4f310bbf97781b9ccc95ae2754224a75b2&to=bd36ced96eabd9b4e028778d058dcb56bb6d86ba&stat=instructions

Apparently wouldInstructionBeTriviallyDead is that expensive. Using !isa<PHINode>(I) && !isa<BinaryOperator>(I) && !isa<GetElementPtrInst>(I)) in keeping with the existing code brings it down to this: http://llvm-compile-time-tracker.com/compare.php?from=051bcd4f310bbf97781b9ccc95ae2754224a75b2&to=abae5ca4dc70078006e4e26095215a732be1f330&stat=instructions%3Au

This is still not as free as I had hoped :/

Address comments

Fix llc invocation

In D142551#4080424, @luismarques wrote:

In D142551#4080298, @luke wrote:

Good idea, I don't have access to a remote on it though. Would someone be able to post a branch with this patch on it?

https://llvm-compile-time-tracker.com/compare.php?from=f84d3dd0fd4ec3be52488e4c29baebb0077b04d4&to=b79315a7071612621ac543cd7595cb487c437a64&stat=instructions:u

Update RISCVInstrInfoVSDPatterns.td too

In D142551#4080233, @luismarques wrote:

It might be a good idea to see the compile-time impact of this with https://llvm-compile-time-tracker.com?

Superseded by D142551

In D142293#4079311, @nikic wrote:

Hm, this feels very hacky. I don't like the idea of adjusting the worklist management for specific patterns.

To suggest a possible alternative, it might be possible to extend isDeadPHICycle(), by making it look through side-effect free instructions, as well as multiple users. This will allow it to remove larger dead phi cycles, so it would not just peel off one phi cycle at a time, but could remove the whole graph at once. (Of course subject to the visitation limit -- with the current one of 16, that would mean we could remove 8 nested phi cycles in one go, which is probably good enough?)

In D142199#4076287, @kiranchandramohan wrote:

The flang website (flang.llvm.org) or flang.llvm.org/docs are built from these. Would you know whether there are changes required are there? https://github.com/llvm/llvm-zorg/blob/main/zorg/buildbot/builders/SphinxDocsBuilder.py

In D142199#4073815, @tstellar wrote:

LGTM. Can you add something to the release notes about this.

Update release notes

Remove unnecessary changes

The codegen changes seem fine to me but perhaps I'm missing something.

Mention GitHub issue in commit message

Don't suppress misc.highlighting_failure warning

Rendered PDF

Remove erroneous include

Address comment

Only scalarize if scalar type is legal

Split out webassembly test into pre-commit test

Check for legal types and equal int/float bit sizes

Add missing checks in half case

Expand to include halfs and conversions to other integer types

Update affected tests

Limit to cases where the shuffle is not just a single non-undef mask element

Don't include test case commit

Update AMDGPU tests with D140537

Rebase and update comments

Address review comments

Rework the approach