InstructionCost is already an optional value, containing an Invalid
state that can be checked with isValid(). There is little point in
returning another optional from getValue(). Most uses do not make use of
it being a std::optional, dereferencing the value directly (either
isValid has been checked previously or the Cost is assumed to be valid).
The one case that does in AMDGPU used value_or which has been replaced
by a isValid() check.

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 getFirstNonPHI use the iterator-returning version.

This patch changes a bunch of call-sites calling getFirstNonPHI to use
getFirstNonPHIIt, which returns an iterator. All these call sites are
where it's obviously safe to fetch the iterator then dereference it. A
follow-up patch will contain less-obviously-safe changes.

We'll eventually deprecate and remove the instruction-pointer
getFirstNonPHI, but not before adding concise documentation of what
considerations are needed (very few).

---------

Co-authored-by: Stephen Tozer <Melamoto@gmail.com>

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).

Loop latches often have a loop-carried dependency, and if they have
several SelectLike instructions in one select group, it is usually
profitable to convert it to branches rather than keep selects.

Building on top of https://github.com/llvm/llvm-project/pull/115489
extend support for binops with SExt operand.

PR: https://github.com/llvm/llvm-project/pull/115879

For conditional increments with sign check conditions like X < 0 or X >= 0,
the compiler may generate code like this:

  %cmp = icmp sgt i64 %1, -1
  %shift = ashr i64 %1, 63
  %j.next = add nsw i64 %j, %shift
  %sel = select i1 %cmp ...

, where %cmp is not in computation but in some other implicit or regular
expressions. This patch allows SelectOptimize pass to recognise these
cases.