Purely for the sake of being idiomatic with other TTI costing routines,
no direct motivation beyond that.

This does not alter much at the moment, but allows const pointers to be
passed as Op0 and Op1, simplifying later patches

Replace "concept based polymorphism" with simpler PImpl idiom.

This pursues two goals:
* Enforce static type checking. Previously, target implementations hid
base class methods and type checking was impossible. Now that they
override the methods, the compiler will complain on mismatched
signatures.
* Make the code easier to navigate. Previously, if you asked your
favorite LSP server to show a method (e.g. `getInstructionCost()`), it
would show you methods from `TTI`, `TTI::Concept`, `TTI::Model`,
`TTIImplBase`, and target overrides. Now it is two less :)

There are three commits to hopefully simplify the review.

The first commit removes `TTI::Model`. This is done by deriving
`TargetTransformInfoImplBase` from `TTI::Concept`. This is possible
because they implement the same set of interfaces with identical
signatures.

The first commit makes `TargetTransformImplBase` polymorphic, which
means all derived classes should `override` its methods. This is done in
second commit to make the first one smaller. It appeared infeasible to
extract this into a separate PR because the first commit landed
separately would result in tons of `-Woverloaded-virtual` warnings (and
break `-Werror` builds).

The third commit eliminates `TTI::Concept` by merging it with the only
derived class `TargetTransformImplBase`. This commit could be extracted
into a separate PR, but it touches the same lines in
`TargetTransformInfoImpl.h` (removes `override` added by the second
commit and adds `virtual`), so I thought it may make sense to land these
two commits together.

Pull Request: https://github.com/llvm/llvm-project/pull/136674

These are not diagnosed because implementations hide the methods of the base class rather than overriding them.
This works as long as a hiding function is callable with the same arguments as the same function from the base class.

Pull Request: https://github.com/llvm/llvm-project/pull/136655

Making `TargetTransformInfo::Model::Impl` `const` makes sure all
interface methods are `const`, in `BasicTTIImpl`, its bases, and in all
derived classes.

Pull Request: https://github.com/llvm/llvm-project/pull/136598

The main change is making `thisT` method `const`, the rest of the
changes is fixing compilation errors (*).

(*) There are two tricky methods, `getVectorInstrCost()` and
`getIntImmCost()`.
They have several overloads; some of these overloads are typically
pulled in to derived classes using the `using` directive, and then
hidden by methods in the derived class.
The compiler does not complain if the hiding methods are not marked as
`const`, which means that clients will use the methods from the base
class. If after this change your target fails cost model tests, this
must be the reason. To resolve the issue you need  to make all hiding
overloads `const`. See the second commit in this PR.

Pull Request: https://github.com/llvm/llvm-project/pull/136575

Enable the vectorizer to access interleaved memory. This means that,
when it's decided to be profitable, the memory accesses can be
vectorized instead of the value being built up by a sequence of
load_lane instructions. This will often increase the vectorization
factor of the loop, leading to significantly better performance.

I run a reasonably large collection of benchmarks and most are not
affected by this change, with most performance changes <1%. But I see a
2.5% speedup for the total run time of TSVC, 1% speedup for SPEC2017
x265, 28% speedup for a ResNet workload and 95% for libyuv. This is
running V8 on an AArch64 box.

Enable the use of partial.reduce.add that we can lower to dot or a tree
of (add (extmul_low_u, extmul_high_u)) for the unsigned case. We support
both v8i16 and v16i8 inputs.

Add inital implementations of these TTI methods for SIMD types. For
casts, The costing covers the free extensions provided by extmul_low as
well as extend_low. For memory operations we consider the use of
load32_zero and load64_zero, as well as full width v128 loads.

This patch moves the `areInlineCompatible` implementation from multiple
subclasses (`AArch64TTIImpl`, `RISCVTTIImpl`, `WebAssemblyTTIImpl`) to
the base class `BasicTTIImpl`. The new implementation checks whether the
callee's target features are a subset of the caller's, enabling
consistent behavior across targets. Subclasses now simply delegate to
the base implementation, reducing code duplication and improving
maintainability.