[InstCombine] (-NSW x) u>= x --> x s>= 0 (PR39480)

Name: (-x) u>= x  -->  x s>= 0
%neg_x = sub nsw i8 0, %x ; %x must not be INT_MIN
%r = icmp uge i8 %neg_x, %x
  =>
%r = icmp sge i8 %x, 0

https://rise4fun.com/Alive/LLHd

https://bugs.llvm.org/show_bug.cgi?id=39480