These selects are dependent on values live into the CHRScope that we
cannot infer anything about, so mark the branch weights unknown. These
selects usually also just get folded down into a icmps, so the profile
information ends up being kind of redundant.

Closes #165240.

This patch replaces LLVM_ATTRIBUTE_UNUSED with [[maybe_unused]].  Note
that this patch adjusts the placement of [[maybe_unused]] to comply
with the C++17 language.

Reapplies #159686

This reverts commit 4f33d7b7a9f39d733b7572f9afbf178bca8da127.

The original landing of this patch had an issue where it would try and
hoist allocas into the entry block that were in the entry block. This
would end up actually moving them lower in the block potentially after
users, resulting in invalid IR.

This update fixes this by ensuring that we are only hoisting static
allocas that have been sunk into a split basic block. A regression test
has been added.

Integration tested using a three stage build of clang with IRPGO
enabled.

This reverts commit a00450944d2a91aba302954556c1c23ae049dfc7.

Looks like this one is actually breaking the buildbots. Reverting the switch back
to IRPGO did not fix things.

ControlHeightReduction will duplicate some blocks and insert phi nodes
in exit blocks of regions that it operates on for any live values. This
includes allocas. Having a lifetime annotation refer to a phi node was
made illegal in 92c55a315eab455d5fed2625fe0f61f88cb25499, which causes
the verifier to fail after CHR.

There are some cases where we might not need to drop lifetime
annotations (usually because we do not need the phi to begin with), but
drop all annotations for now to be conservative.

Fixes #159621.

We can use *Set::insert_range to collapse:

  for (auto Elem : Range)
    Set.insert(E);

down to:

  Set.insert_range(Range);

In some cases, we can further fold that into the set declaration.

DenseSet, SmallPtrSet, SmallSet, SetVector, and StringSet recently
gained C++23-style insert_range.  This patch replaces:

  Dest.insert(Src.begin(), Src.end());

with:

  Dest.insert_range(Src);

This patch does not touch custom begin like succ_begin for now.

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).