InstCombine: Combine (X+cst) < 0 --> X < -cst

InstCombine: Combine (X+cst) < 0 --> X < -cst

This can happen pretty often in code that looks like:
int foo = bar - 1;
if (foo < 0)
  do stuff

In this case, bar < 1 is an equivalent condition.

This transform requires that the add instruction be annotated with nsw.

llvm-svn: 221045