C's Annex F specifies that atan +/-0.0 returns the input value;
however, this behavior is optional and host C libraries may behave
differently. This change applies the Annex F behavior to constant
folding by LLVM.

Ref:
https://pubs.opengroup.org/onlinepubs/9799919799/functions/atan.html

ReadDataFromGlobal() did not handle reads from the padding of types (in
the sense of type store size != type alloc size, rather than struct
padding).

Return zero in that case.

Fixes https://github.com/llvm/llvm-project/issues/144279.

While there remove an unnecessary dyn_cast from Constant to Constant.
Reverse a branch condition into an early out to reduce nesting.

The change adds folding for 4 vector intrinsics: `interleave2`,
`deinterleave2`, `vector_extract` and `vector_insert`. For the last 2
intrinsics the change does not use `ShuffleVector` fold mechanism as
it's much simpler to construct result vector explicitly.

Testcase from https://github.com/llvm/llvm-project/pull/143416 is
causing the AIX bot to be red. XFAIL for now till issue can be resolved.

Fixes #143360

As noted in
https://github.com/llvm/llvm-project/pull/141821#issuecomment-2917328924,
whilst we currently constant fold intrinsics of fixed-length vectors via
their scalar counterpart, we don't do the same for scalable vectors.

This handles the scalable vector case when the operands are splats.

One weird snag in ConstantVector::getSplat was that it produced a undef
if passed in poison, so this also contains a fix by checking for
PoisonValue before UndefValue.

Add constant-folding support for the maximumnum and minimumnum
intrinsics, and extend the tests to show the qnan vs snan behavior
differences between maxnum/maximum/maximumnum.

https://github.com/llvm/llvm-project/pull/130742#discussion_r1993055149

Previously only fixed vector splats were handled. This adds supports for
scalable vectors too by allowing ConstantExpr splats.

We need to add the extra V->getType()->isVectorTy() check because a
ConstantExpr might be a scalar to vector bitcast.

By allowing ConstantExprs this also allow fixed vector ConstantExprs to
be folded, which causes the diffs in
llvm/test/Analysis/ValueTracking/known-bits-from-operator-constexpr.ll
and llvm/test/Transforms/InstSimplify/ConstProp/cast-vector.ll. I can
remove them from this PR if reviewers would prefer.

Fixes #132922