This hook replaces inline asm with LLVM intrinsics. It was intended to
match inline assembly implementations of bswap in libc headers and
replace them more optimizable implementations.

At this point, it has outlived its usefulness (see
https://github.com/llvm/llvm-project/issues/156571#issuecomment-3247638412),
as libc implementations no longer use inline assembly for this purpose.

Additionally, it breaks the "black box" property of inline assembly,
which some languages like Rust would like to guarantee.

Fixes https://github.com/llvm/llvm-project/issues/156571.

This patch replaces SmallSet<T *, N> with SmallPtrSet<T *, N>.  Note
that SmallSet.h "redirects" SmallSet to SmallPtrSet for pointer
element types:

  template <typename PointeeType, unsigned N>
class SmallSet<PointeeType*, N> : public SmallPtrSet<PointeeType*, N>
{};

We only have 140 instances that rely on this "redirection", with the
vast majority of them under llvm/. Since relying on the redirection
doesn't improve readability, this patch replaces SmallSet with
SmallPtrSet for pointer element types.

Compare sinking is selectable based on the result of
hasMultipleConditionRegisters. This function is too coarse grained by
not taking into account the differences between scalar and vector
compares. This PR extends the interface to take an EVT to allow finer
control.
    
The new interface is used by AArch64 to disable sinking of scalable
vector compares, but with isProfitableToSinkOperands updated to maintain
the cases that are specifically tested.

Fix redundant LEA: https://godbolt.org/z/34xEYE818

Closes https://github.com/llvm/llvm-project/issues/150611.

This is one of the final remaining debug-intrinsic specific codepaths
out there, and pieces of cross-LLVM infrastructure to do with debug
intrinsics.

This is another prune of dead code -- we never generate debug intrinsics
nowadays, therefore there's no need for these codepaths to run.

---------

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

With the advent of intrinsic-less debug-info, we no longer need to
scatter calls to getPrevNonDebugInstruction around the codebase. Remove
most of them -- there are one or two that have the "SkipPseudoOp" flag
turned on, however they don't seem to be in positions where skipping
anything would be reasonable.

There are no longer debug-info instructions, thus we don't need this
skipping. Horray!

Follow up on #139303