Making the choice more clear from the API name, otherwise it'd be very easy for one to just "not bother" with the `MDFrom`​, especially since it is optional and follows the optional `Name`​ - but this time we'd have a harder time detecting it's effectivelly dropped metadata.

These select instructions are created from non-branching instructions,
so their branch weights are unknown.

Tracking issue: #147390

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.

Infer the 'exact' flag on an 'lshr' or 'ashr' instruction when the shift
amount is computed via a 'cttz' intrinsic on the same operand.
 
Proof: https://alive2.llvm.org/ce/z/CQR2PG

Fixes #131444.

#121386 Introduced cttz intrinsics which caused a regression where
vscale/vscale divisions could no longer be constant folded.

This fold was suggested as a fix in #126411.
https://alive2.llvm.org/ce/z/gWbtPw

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 following pattern: `(C2 << X) << C1` will usually be transformed
into `(C2 << C1) << X`, essentially swapping `X` and `C1`.

However, this should only be done when `C1` is an immediate constant,
otherwise thiscan lead to both constants being swapped forever.

This fixes #118798.

Alive2: https://alive2.llvm.org/ce/z/kwvTFn
Closes #107228.

`ashr iN (X-1)&~X, N-1` also exists. See
https://github.com/dtcxzyw/llvm-opt-benchmark/issues/1274.

Proof: https://alive2.llvm.org/ce/z/4diUqN

---------

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