Once we simplified a phi node, we never updated the definition it points
to, which meant that it could become out of date if that definition were
also simplified, and we didn't check that when rewriting sources. That
could happen when there are multiple nested loops with phi nodes at the
header.

Fix it by updating the phi's pointer. Since we always update sources
after visiting the definition it points to, when we go to rewrite a
source, if that source points to a simplified phi, the phi's pointer
can't be pointing to a simplified phi because we already visited the phi
earlier in the pass and updated it, or else it's been simplified in the
meantime and this isn't the last pass. This way we don't need to
keep recursing when rewriting sources.