Use carry flag to reduce instructions

NOTE:
(1) Fixnum's LSB is always 1.
    It means you can always run `x - 1` without overflow.
(2) Of course `z = x + (y-1)` may overflow.
    Now z's LSB is always 1, and the MSB of true result is also 1.
    You can get true result in long as `(1<<63)|(z>>1)`,
    and it equals to `(z<<63)|(z>>1)` == `ror(z)`.

GCC and Clang have __builtin_add_ovewflow:
* https://gcc.gnu.org/onlinedocs/gcc/Integer-Overflow-Builtins.html
* https://clang.llvm.org/docs/LanguageExtensions.html#checked-arithmetic-builtins

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57506 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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