Having a finite Depth (or recursion limit) for computeKnownBits is very
limiting, but is currently a load-bearing necessity, as all KnownBits
are recomputed on each call and there is no caching. As a prerequisite
for an effort to remove the recursion limit altogether, either using a
clever caching technique, or writing a easily-invalidable KnownBits
analysis, make the Depth argument in APIs in ValueTracking uniformly the
last argument with a default value. This would aid in removing the
argument when the time comes, as many callers that currently pass 0
explicitly are now updated to omit the argument altogether.

Closes #138691

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).

This patch introduces a new method:

void Vectorizer::mergeEquivalenceClasses(EquivalenceClassMap &EQClasses)
const;

The method is called at the end of
Vectorizer::collectEquivalenceClasses() and is needed to merge
equivalence classes that differ only by their underlying objects (UO1
and UO2), where UO1 is 1-level-indirection underlying base for UO2. This
situation arises due to the limited lookup depth used during the search
of underlying bases with llvm::getUnderlyingObject(ptr).

Using any fixed lookup depth can result into creation of multiple
equivalence classes that only differ by 1-level indirection bases.

The new approach merges equivalence classes if they have adjacent bases
(1-level indirection). If a series of equivalence classes form ladder
formed of 1-step/level indirections, they are all merged into a single
equivalence class. This provides more opportunities for the load-store
vectorizer to generate better vectors.

---------

Signed-off-by: Klochkov, Vyacheslav N <vyacheslav.n.klochkov@intel.com>

Reverts llvm/llvm-project#114501, due to the following failure:
https://lab.llvm.org/buildbot/#/builders/55/builds/4171

This patch introduces a new method:
void Vectorizer::mergeEquivalenceClasses(EquivalenceClassMap &EQClasses)
const

The method is called at the end of
Vectorizer::collectEquivalenceClasses() and is needed to merge
equivalence classes that differ only by their underlying objects (UO1
and UO2), where UO1 is 1-level-indirection underlying base for UO2. This
situation arises due to the limited lookup depth used during the search
of underlying bases with llvm::getUnderlyingObject(ptr).

Using any fixed lookup depth can result into creation of multiple
equivalence classes that only differ by 1-level indirection bases.

The new approach merges equivalence classes if they have adjacent bases
(1-level indirection). If a series of equivalence classes form ladder
formed of 1-step/level indirections, they are all merged into a single
equivalence class. This provides more opportunities for the load-store
vectorizer to generate better vectors.

---------

Signed-off-by: Klochkov, Vyacheslav N <vyacheslav.n.klochkov@intel.com>

This reverts commit c46b41aaa6eaa787f808738d14c61a2f8b6d839f.

Multiple tests time out, either due to performance hit (see comment) or
a cycle.

Forbid load-load cycles which would crash LoadStoreVectorizer when
reordering instructions.

Fixes #37865.