[X86] Enable constexpr on LZCNT and BMI intrinsics

[RKSimon]

The x86 specific LZCNT and BMI intrinsics in lzcntintrin.h and bmiintrin.h are all capable of being converted to constexpr

We did something similar for the POPCNT intrinsics here: https://reviews.llvm.org/D86229 / cff0db0

• Update __builtin_ia32_* intrinsics to allow handling in ExprConstant
• Update headers - ensuring they work for c, pre-C++11 and later C++ builds
• Test coverage - including the macro variants of some of the methods (e.g. _tzcnt_u32 for __tzcnt_u32)

[llvmbot]

@llvm/issue-subscribers-backend-x86

Author: Simon Pilgrim (RKSimon)

The x86 specific LZCNT and BMI intrinsics in lzcntintrin.h and bmiintrin.h are all capable of being converted to constexpr

We did something similar for the POPCNT intrinsics here: https://reviews.llvm.org/D86229 / cff0db0

• Update __builtin_ia32_* intrinsics to allow handling in ExprConstant
• Update headers - ensuring they work for c, pre-C++11 and later C++ builds
• Test coverage - including the macro variants of some of the methods (e.g. _tzcnt_u32 for __tzcnt_u32)

[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. In the comments of the issue, request for it to be assigned to you.
2. Fix the issue locally.
3. 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.
4. Create a Git commit.
5. Run git clang-format HEAD~1 to format your changes.
6. Open a pull request to the upstream repository on GitHub. Detailed instructions can be found in GitHub's documentation.

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 x86 specific LZCNT and BMI intrinsics in lzcntintrin.h and bmiintrin.h are all capable of being converted to constexpr

We did something similar for the POPCNT intrinsics here: https://reviews.llvm.org/D86229 / cff0db0

• Update __builtin_ia32_* intrinsics to allow handling in ExprConstant
• Update headers - ensuring they work for c, pre-C++11 and later C++ builds
• Test coverage - including the macro variants of some of the methods (e.g. _tzcnt_u32 for __tzcnt_u32)

[aniplcc]

I'm interested in taking this one.

[RKSimon]

@aniplcc reverse-ping?

[RKSimon]

#110499 added support for LZCNT/TZCNT x86 intrinsics

[RKSimon]

#110581 added support for BMI1 intrinsics

[RKSimon]

#110515 added support for TBM intrinsics

[RKSimon]

#110508 / #110535 / #110654 added support for BMI2 BZHI / PDEP+PEXT / MULX intrinsics