- Use `ArrayRef` instead of `SmallVector` reference in a few places.
- Drop redundant `llvm::` in a few places.

After f9250401ef120a4605ad67bb43d3b25500900498, this function is
tail recursive so it was straightforward to convert this to iteratively
walk the linkd list.

ContractNodes recursively walks forward through a linked list. During
this recursion, Matchers are combined into other Matchers.

Previously the formation of MoveSiblingMatcher was after the
recursive call so it occurred as we were unwinding. If a
MoveSiblingMatcher was formed, we would recursively walk forward
to the end of the linked list again which isn't efficient.

To make this more efficient, move the formation of MoveSiblingMatcher
to the forward pass. Add additional rules to unfold MoveSiblingMatcher
if it would be more efficient to use CheckChildType, CheckChildInteger,
CheckChildSame, etc.

As an added benefit, this makes the function tail recursive which
the compiler can better optimize.

It appears that CheckOpcode is already emitted before CheckType so
this hasn't been doing anything on any in tree targets.

The loop at the top of FactorNodes creates additional variables to deal
with needing to use a pointer to a unique_ptr instead of a reference.
Encapsulate this to its own function for better scoping.

This also allows us to directly skip this loop when we already know we
have a ScopeMatcher.

The code that moves CheckOpcode before CheckType/CheckChildType/RecordDwith
was running after ContractNodes started unwinding its recursion. If a
move occurs we would start a new recursion going forward
through the list again. I don't believe this can lead to any new
combines so it was just wasted work.

This patch moves the code earlier so it doesn't start a new recursion.