[NFCI][SYCL] Refactor reductions implementations

[aelovikov-intel]

The purpose of this change is

1. Make it clear that sycl::range version always delegates to some
   sycl::nd_range implementation.

2. Flatten all existing implementations. That required splitting
   reduction_parallel_for_basic_impl depending on
   Reduction::has_fast_reduce. However, I also inlined its helper
   routines (that now had unambigious branches for "if
   constexpr (Reduction::*)") so the ratio between duplicate/unique code
   isn't bad at all.

3. Make a unique dispatching entry and have all the implementations
   provide the same interface. I plan to use it in unit-tests to bypass
   the dispatch and test all the implementation directly (when
   applicable based on nd_range/BinOp/HW/etc.).

I'd say 90% of the change is straightforward code movement with the
exceptions of

• Inlining described above
• Simplifying __sycl_reduction_kernel helper enabled by this change
• Factoring out forward declarations in handler.hpp into a separate
  reduction_fordward.hpp
• Rewriting dispatcher routine - caused by changes in the interfaces,
  not logic updates.

[bader]

// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception

License header is missing.

[aelovikov-intel]

Done

[steffenlarsen]

LGTM. Just a minor nit.

[aelovikov-intel]

Failures:

SYCL :: Regression/device_num.cpp
SYCL :: regression/tanh_fix_test.cpp  / on ESIMD

seem to be unrelated (e.g., the same are reported on https://github.com/intel/llvm/actions/runs/3430654188/jobs/5718451880). @intel/llvm-gatekeepers can this be merged in?

[pvchupin]

@aelovikov-intel, please find the cause of fails. @againull please don't merge until cause is presented and owner identified.
Also there are post-commit fails: https://github.com/intel/llvm/actions/runs/3432868067

[againull]

@aelovikov-intel, please find the cause of fails. @againull please don't merge until cause is presented and owner identified. Also there are post-commit fails: https://github.com/intel/llvm/actions/runs/3432868067

Got it, sorry.

[aelovikov-intel]

Also there are post-commit fails: https://github.com/intel/llvm/actions/runs/3432868067

#7345 should hopefully address those warnings - my local build doesn't have them for some reason.

[steffenlarsen]

SYCL :: Regression/device_num.cpp - Reported to owner in intel/llvm-test-suite#1354 (comment) with fix up in #7336.
SYCL :: regression/tanh_fix_test.cpp - Reported to owner in intel/llvm-test-suite#1361 (comment) with fix in intel/llvm-test-suite#1372.