[InstCombine] Canonicalize phi order for newly inserted nodes

When new phi nodes are inserted at the start of the block, the
order of these does not get canonicalized, as we pick the first
phi node to canonicalize towards (and the other phi nodes may have
already been visited). This results in phi nodes not being
deduplicated and thus a fix-point verification failure.

Fix this by remembering the predecessors of the first phi node
we encounter -- this will usually result in the same order we
picked previously. I also considered using predecessors() order
instead, but that causes substantial test fallout. Additionally
the predecessors() order tends to be the reverse of the "natural"
order.

Fixes https://github.com/llvm/llvm-project/issues/46688.