(REVERSIBLE_CC_MODE): Add default definition.

(reversible_comparison_p): REVERSIBLE_CC_MODE.

(subst): Always simplify true and false arms if if_then_else_cond succeeds.
(subst, case IF_THEN_ELSE): More canonicalizations; delete some redundant
tests.
(if_then_else_cond): Canonicalize (OP (mult COMP A) (mult REV_COMP B)).
(gen_binary): Simplify (REL_OP (compare X Y) 0) here.

From-SVN: r6540