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.

Directly modeled after what we do for vector.reverse, but with
restrictions on EVL and mask added.

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.

I noticed this when a sqrt produced by VectorCombine with a poison
operand wasn't getting folded away to poison.

Most intrinsics in general could probably be folded to poison if one of
their arguments are poison too. Are there any exceptions to this we need
to be aware of?