[X86] Remove X86ISD::VSHLDV/VSHRDV and use ISD::FSHL/FSHR opcodes directly

[RKSimon]

The main difficulty is that the VSHRDV instruction requires the LO/HI operands commuting in the isel patterns, so only attempt this if you know your way around tablegen / isel.

[llvmbot]

Hi!

This issue may be a good introductory issue for people new to working on LLVM. If you would like to work on this issue, your first steps are:

1. Check that no other contributor is working on this issue. If someone is assigned to the issue or claimed to be working on it, ping the person. After one week without a response, the assignee may be changed.
2. Leave a comment indicating that you are working on the issue, or just create a pull request after following the steps below. Mention this issue in the description of the pull request.
3. Fix the issue locally.
4. Run the test suite locally. Remember that the subdirectories under test/ create fine-grained testing targets, so you can e.g. use make check-clang-ast to only run Clang's AST tests.
5. Create a Git commit.
6. Run git clang-format HEAD~1 to format your changes.
7. Open a pull request to the upstream repository on GitHub. Detailed instructions can be found in GitHub's documentation. Mention this issue in the description of the pull request.

If you have any further questions about this issue, don't hesitate to ask via a comment in the thread below.

[llvmbot]

@llvm/issue-subscribers-good-first-issue

Author: Simon Pilgrim (RKSimon)

The main difficulty is that the VSHRDV instruction requires the LO/HI operands commuting in the isel patterns, so only attempt this if you know your way around tablegen / isel.

[llvmbot]

@llvm/issue-subscribers-backend-x86

Author: Simon Pilgrim (RKSimon)

The main difficulty is that the VSHRDV instruction requires the LO/HI operands commuting in the isel patterns, so only attempt this if you know your way around tablegen / isel.

[TianYe717]

I'd be interested in taking this on. I have some experience with TableGen/ISel and would like to give it a try