Some of the changes in the patch include:

1. Using iterators instead of instruction pointers when applicable.
2. Modifying Polly functions to accept iterators instead of inst
pointers.
3. Updating API usages such as use begin instead of front.

If the preload condition is a constant, ExprBuilder::create returns an
integer of the native integer while an i1 is expected. Cast the result
to i1 if that happens.

Fixes #123932

Patch created using the following command line:
```bash
codespell polly --skip="*.pdf,polly/lib/External/*" --write-changes \
  --ignore-words-list=couter,createor,distribues,doble,identty,indention,indx,olt,ore,padd,sais,te,theses
```

This patch introduces the initial implementation for annotating loops
created by Polly. Polly generates RunTimeChecks (RTCs), which result in
loop versioning. Specifically, the loop created by Polly is executed
when the RTCs pass, otherwise, the original loop is executed.

This patch adds the "llvm.loop.vectorize.enable" metadata, setting it to
true for loops created by Polly. Disabling vectorization for the original
fallback loop is already merged in #119188.

This behavior is controlled by the 'polly-annotate-metadata-vectorize'
flag, and the annotations are applied only when this flag is enabled.
This flag is set to false by default.

NOTE: This commit is initial patch in effort to make polly interact with
Loop Vectorizer via metadata.

---------

Co-authored-by: Michael Kruse <github@meinersbur.de>

The patch #102460 already implements separate DT/LI/SE for parallel sub
function. Crashes have been reported while region generator tries using
oringinal function's DT while creating new parallel sub function due to
checks in #101198. This patch aims at fixing those cases by switching
the DT/LI while generating parallel function using Region Generator.

Fixes #117877

DominatorTree, LoopInfo, and ScalarEvolution are function-level analyses
that expect to be called only on instructions and basic blocks of the
function they were original created for. When Polly outlined a parallel
loop body into a separate function, it reused the same analyses seemed
to work until new checks to be added in #101198.

This patch creates new analyses for the subfunctions. GenDT, GenLI, and
GenSE now refer to the analyses of the current region of code. Outside
of an outlined function, they refer to the same analysis as used for the
SCoP, but are substituted within an outlined function.

Additionally to the cross-function queries of DT/LI/SE, we must not
create SCEVs that refer to a mix of expressions for old and generated
values. Currently, SCEVs themselves do not "remember" which
ScalarEvolution analysis they were created for, but mixing them is just
as unexpected as using DT/LI across function boundaries. Hence
`SCEVLoopAddRecRewriter` was combined into `ScopExpander`.
`SCEVLoopAddRecRewriter` only replaced induction variables but left
SCEVUnknowns to reference the old function. `SCEVParameterRewriter`
would have done so but its job was effectively superseded by
`ScopExpander`, and now also `SCEVLoopAddRecRewriter`. Some issues
persist put marked with a FIXME in the code. Changing them would
possibly cause this patch to be not NFC anymore.

The base concept is same as existing reduction algorithm where we get
the list of candidate pairs <store,load>. But the existing algorithm
works only if there is single binary operation between the load and
store.
Example sum += a[i];

This algorithm extends to work with more than single binary operation as
well. It is implemented using data flow reduction detection on basic
block level. We propagate the loads, the number of times the load is
used(flows into instruction) and binary operation performed until we
reach a store.

Example sum += a[i] + b[i];
```
sum(Ld)     a[i](Ld)
      \  +  /
        tmp    b[i](Ld)
           \ + /
            sum(St)
```

In the above case the candidate pairs are formed by associating sum with
all of its load inputs which are sum, a[i] and b[i]. Then check
functions are used to filter a valid reduction pair ie {sum,sum}.

---------

Co-authored-by: Michael Kruse <github@meinersbur.de>

This patch simplifies instruction creation by replacing all overloads of
instruction constructors/Create methods that are identical other than
the Instruction *InsertBefore/BasicBlock *InsertAtEnd/BasicBlock::iterator
InsertBefore argument with a single version that takes an InsertPosition
argument. The InsertPosition class can be implicitly constructed from
any of the above, internally converting them to the appropriate
BasicBlock::iterator value which can then be used to insert the
instruction (or to not insert it if an invalid iterator is passed).

The upshot of this is that code will be deduplicated, and all callsites
will switch to calling the new unified version without any changes
needed to make the compiler happy. There is at least one exception to
this; the construction of InsertPosition is a user-defined conversion,
so any caller that was already relying on a different user-defined
conversion won't work. In all of LLVM and Clang this happens exactly
once: at clang/lib/CodeGen/CGExpr.cpp:123 we try to construct an alloca
with an AssertingVH<Instruction> argument, which must now be cast to an
Instruction* by using `&*`. If this is more common elsewhere, it could
be fixed by adding an appropriate constructor to InsertPosition.