Suggestion for a fix to Bug middle-end/20177

[Richard Kenner]

    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.