This patch removes canRotateDeoptimizingLatchExit check from loop roate
and connected multi rotation option for loop roate (-loop-rotate-multi
option).

The heuristic in canRotateDeoptimizingLatchExit returns true if any of
the loop exits are non-deoptimizing. This means if the loop has multiple
deopt exits, then without multi-rotate, we may still end up having deopt
exit at the latch. This multi-rotate option is introduced but it is off
by default. 

We have not seen any improvements downstream as well, where
we have frequent number of deoptimizations in IR. Since the original
heuristic without the multi-rotate effectively ends up being "rotate
under some conditions", but the loop may still not be in the form we
want, we should remove the heuristic and multi-rotate framework entirely

Note that the diff is big, but it's just removal of while loop and
indentation change.

After this patch I would like to continue here and propose adding a
computability check for exit count, but that will be in a separate PR.

Requests for review: @annamthomas @fhahn @davemgreen

Co-authored-by: Marek Sedlacek <msedlacek@azul.com>

This is one of the final remaining debug-intrinsic specific codepaths
out there, and pieces of cross-LLVM infrastructure to do with debug
intrinsics.

This is another prune of dead code -- we never generate debug intrinsics
nowadays, therefore there's no need for these codepaths to run.

---------

Co-authored-by: Nikita Popov <github@npopov.com>

Seeing how we can't generate any debug intrinsics any more: delete a
variety of codepaths where they're handled. For the most part these are
plain deletions, in others I've tweaked comments to remain coherent, or
added a type to (what was) type-generic-lambdas.

This isn't all the DbgInfoIntrinsic call sites but it's most of the
simple scenarios.

Co-authored-by: Nikita Popov <github@npopov.com>

This flag was used to let us incrementally introduce debug records
into LLVM, however everything is now using records. It serves no
purpose now, so delete it.

RFC: https://discourse.llvm.org/t/rfc-improving-is-stmt-placement-for-better-interactive-debugging/82668

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).

The `poison` values are used to substitute debug information of values
moved from the original header into the preheader that are no longer
available in the former.