In C++17, static constexpr members are implicitly inline, so they no
longer require an out-of-line definition.

Identified with readability-redundant-declaration.

In [1], Nikita Popov spotted that two BitCast operations are not needed
with opaque pointers. So remove these two BitCast operations and adjust
corresponding comments as well.

  [1] https://github.com/llvm/llvm-project/pull/130722

Co-authored-by: Yonghong Song <yonghong.song@linux.dev>

Alexei reported a bpf selftest failure with recent llvm for bpf prog
file progs/arena_spin_lock.c. The failure only happens when clang is
built with cmake option LLVM_ENABLE_ASSERTIONS=ON.

The error message looks like:
```
 clang: /home/yhs/work/yhs/llvm-project/llvm/lib/IR/Instructions.cpp:3460:
   llvm::BitCastInst::BitCastInst(Value *, Type *, const Twine &, InsertPosition):
   Assertion `castIsValid(getOpcode(), S, Ty) && "Illegal BitCast"' failed.
```
Further investigation shows that the problem is triggered in
  BPF/BPFAbstractMemberAccess.cpp
for code
```
  auto *BCInst =
      new BitCastInst(Base, PointerType::getUnqual(BB->getContext()));
```
For the above BitCastInst, Since 'Base' has non-zero AddrSapce, the
compiler expects the type also has the same AddrSpace. But the above
PointerType::getUnqual(...) does not have AddrSpace and hence causes the
assertion failure.

Providing the proper AddrSpace for the BitCast type fixed the issue.

Co-authored-by: Yonghong Song <yonghong.song@linux.dev>

As part of the "RemoveDIs" project, BasicBlock::iterator now carries a
debug-info bit that's needed when getFirstNonPHI and similar feed into
instruction insertion positions. Call-sites where that's necessary were
updated a year ago; but to ensure some type safety however, we'd like to
have all calls to moveBefore use iterators.

This patch adds a (guaranteed dereferenceable) iterator-taking
moveBefore, and changes a bunch of call-sites where it's obviously safe
to change to use it by just calling getIterator() on an instruction
pointer. A follow-up patch will contain less-obviously-safe changes.

We'll eventually deprecate and remove the instruction-pointer
insertBefore, but not before adding concise documentation of what
considerations are needed (very few).

Note that PointerUnion::{is,get,dyn_cast} have been soft deprecated in
PointerUnion.h:

  // FIXME: Replace the uses of is(), get() and dyn_cast() with
  //        isa<T>, cast<T> and the llvm::dyn_cast<T>

Rename the function to reflect its correct behavior and to be consistent
with `Module::getOrInsertFunction`. This is also in preparation of
adding a new `Intrinsic::getDeclaration` that will have behavior similar
to `Module::getFunction` (i.e, just lookup, no creation).

These are the last remaining "trivial" changes to passes that use
Instruction pointers for insertion. All of this should be NFC, it's just
changing the spelling of how we identify a position.

In one or two locations, I'm also switching uses of getNextNode etc to
using std::next with iterators. This too should be NFC.

---------

Merged by: Stephen Tozer <stephen.tozer@sony.com>