I'd reverted this in 6c7805d5d1 after a bad stage. Original commit
messsage follows:

[NFC][RemoveDIs] Bulk update utilities to insert with iterators

As part of the RemoveDIs project we need LLVM to insert instructions using
iterators wherever possible, so that the iterators can carry a bit of
debug-info. This commit implements some of that by updating the contents of
llvm/lib/Transforms/Utils to always use iterator-versions of instruction
constructors.

There are two general flavours of update:
 * Almost all call-sites just call getIterator on an instruction
 * Several make use of an existing iterator (scenarios where the code is
   actually significant for debug-info)
The underlying logic is that any call to getFirstInsertionPt or similar
APIs that identify the start of a block need to have that iterator passed
directly to the insertion function, without being converted to a bare
Instruction pointer along the way.

I've also switched DemotePHIToStack to take an optional iterator: it needs
to take an iterator, and having a no-insert-location behaviour appears to
be important. The constructors for ICmpInst and FCmpInst have been updated
too. They're the only instructions that take block _references_ rather than
pointers for certain calls, and a future patch is going to make use of
default-null block insertion locations.

All of this should be NFC.

This reverts commit 3fda50d3915b2163a54a37b602be7783a89dd808.

Apparently I've missed a hunk while staging this; will back out for now.

Picked up here: https://lab.llvm.org/buildbot/#/builders/139/builds/60429/steps/6/logs/stdio

As part of the RemoveDIs project we need LLVM to insert instructions using
iterators wherever possible, so that the iterators can carry a bit of
debug-info. This commit implements some of that by updating the contents of
llvm/lib/Transforms/Utils to always use iterator-versions of instruction
constructors.

There are two general flavours of update:
 * Almost all call-sites just call getIterator on an instruction
 * Several make use of an existing iterator (scenarios where the code is
   actually significant for debug-info)
The underlying logic is that any call to getFirstInsertionPt or similar
APIs that identify the start of a block need to have that iterator passed
directly to the insertion function, without being converted to a bare
Instruction pointer along the way.

I've also switched DemotePHIToStack to take an optional iterator: it needs
to take an iterator, and having a no-insert-location behaviour appears to
be important. The constructors for ICmpInst and FCmpInst have been updated
too. They're the only instructions that take block _references_ rather than
pointers for certain calls, and a future patch is going to make use of
default-null block insertion locations.

All of this should be NFC.

This PR fixes #74572 where the freeze instruction could be found twice
by the pass CanonicalizeFreezeInLoops, and then the compiling may crash
in second removal since the instruction has already gone.

This patch adds #include "llvm/ADT/SmallSet.h" to a couple of files
that are relying on transitive includes of SmallSet.h.  It in turn
unblocks the removal of unnecessary includes of llvm/ADT/SmallSet.h in
several other files.

Estimation on the impact on preprocessor output:
before: 1065307662
after:  1064800684

Discourse thread: https://discourse.llvm.org/t/include-what-you-use-include-cleanup
Differential Revision: https://reviews.llvm.org/D120741

Looking for users of IVUsers, this was a false positive. Only LSR
uses IVUsers.

This patch updates `isGuaranteedNotToBeUndefOrPoison` to use `llvm.assume`'s `noundef` operand bundle.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D89219

Move implicit include dependencies down to header/source files.

NFC.