REG_NOTE (NONNEG)
REG_NOTE (NO_CONFLICT)
REG_NOTE (UNUSED)
mustn't be copied
describe a fact about other instructions so this
may change if copied.
Tricky. Often UNUSED means that we're allocating a psuedo for some temporary
which we know isn't used.
REG_NOTE (CC_SETTER)
REG_NOTE (CC_USER)
I suppose that using CC0 is possible only in branch instructions (correct
me if I am wrong)
It can also be used in "scc" insns.
