The following avx512 intrinsics were removed:
- `__builtin_ia32_vplzcntd_128`
- `__builtin_ia32_vplzcntd_256`
- `__builtin_ia32_vplzcntd_512`
- `__builtin_ia32_vplzcntq_128`
- `__builtin_ia32_vplzcntq_256`
- `__builtin_ia32_vplzcntq_512`

Users of the removed intrinsics (e.g. `_mm512_lzcnt_epi64`,
`_mm_lzcnt_epi32`) are now relying on `__builtin_elementwise_ctlz` and
are marked as `constexpr`.

Fixes: #154279

Replaces the XOP/AVX512 per-element rotation/funnel shift builtins with the generic __builtin_elementwise_fshl/fshr

We still have uniform immediate variants to handle next.

Part of #153152

The following intrinsics were replaced by a combination of
`__builtin_shufflevector` and `__builtin_convertvector`:
- `__builtin_ia32_vcvtph2ps`
- `__builtin_ia32_vcvtph2ps256`

Fixes #152749

The following intrinsics were replaced by `__builtin_elementwise_fma`:
- `__builtin_ia32_vfmaddps(256)`
- `__builtin_ia32_vfmaddpd(256)`
- `__builtin_ia32_vfmaddph(256)`
- `__builtin_ia32_vfmaddbf16(128 | 256 | 512)`

All the aforementioned `__builtin_ia32_vfmadd*` intrinsics are
equivalent to a `__builtin_elementwise_fma`, so keeping them is an
unnecessary indirection.

Fixes [#152461](https://github.com/llvm/llvm-project/issues/152461)

---------

Co-authored-by: Simon Pilgrim <llvm-dev@redking.me.uk>

This reverts commit 83ff9d4a34b1e579dd809759d13b70b8837f0cde.

Don't change the builtin signature of _mm_prefetch this time.

Remove superfluous header files after refactor in #132252

clang/lib/CodeGen/CGBuiltin.cpp is over 1MB long (>23k LoC), and can
take minutes to recompile (depending on compiler and host system) when
modified, and 5 seconds for clangd to update for every edit. Splitting
this file was discussed in this thread:

   https://discourse.llvm.org/t/splitting-clang-s-cgbuiltin-cpp-over-23k-lines-long-takes-1min-to-compile/

and the idea has received a number of +1 votes, hence this change.