cmd/compile: move flagalloc op splitting to rewrite rules
Flagalloc has the unenviable task of splitting
flag-generating ops that have been merged with loads
when the flags need to "spilled" (i.e. regenerated).
Since there weren't very many of them, there was a hard-coded list
of ops and bespoke code written to split them.
This change migrates load splitting into rewrite rules,
to make them easier to maintain.