This PR addresses issues related to the `amdgcn_cs_chain` intrinsic:

1. Ensures the intrinsic's special attribute and calling convention
requirements are not violated by the mutator.
2. Enforces the necessary unreachable statement following this type of
intrinsic, preventing the fuzzer from generating invalid code.

Note that llvm::any_of can accommodate std::begin(Range), not just
Range.begin().

This PR prevents the IRMutator from incorrectly calling functions that
have ABI attributes, otherwise the mutations introduce UB.

The mutator can generate calls with undefined behavior because it is not
matching the calling conventions.

The current implementation can generate invalid IR due to illegal use of
CCs. This matches the behaviour of the IRVerifier.

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).

These are the final few places in LLVM that use instruction pointers to
insert instructions -- use iterators instead, which is needed for
debug-info correctness in the future. Most of this is a gentle
scattering of getIterator calls or not deref-then-addrofing iterators.
libfuzzer does require a storage change to keep built instruction
positions in a container though. The unit-test changes are very
straightforwards.

This leaves us in a position where libfuzzer can't fuzz on either of
debug-info records, however I don't believe that fuzzing of debug-info
is in scope for the library.

Move PassInstrumentationAnalysis into PassInstrumentation.h and stop
including it in PassManager.h (effectively inverting the direction of
the dependency).

Most places using PassManager are not interested in PassInstrumentation,
and we no longer have any uses of it in PassManager.h itself (only in
PassManagerImpl.h).