[InstCombine] add folds for icmp + sub (PR36969)

(A - B) >u A --> A <u B
C <u (C - D) --> C <u D

https://rise4fun.com/Alive/e7j

Name: ugt
  %sub = sub i8 %x, %y
  %cmp = icmp ugt i8 %sub, %x
=>
  %cmp = icmp ult i8 %x, %y
  
Name: ult
  %sub = sub i8 %x, %y
  %cmp = icmp ult i8 %x, %sub
=>
  %cmp = icmp ult i8 %x, %y

This should fix:
https://bugs.llvm.org/show_bug.cgi?id=36969

llvm-svn: 329011