So SCEV cannot simplify that expression to -1 because of overflow issues (take e.g. src=0x7FFF..).
If you compile with 64 bits integers, then the sexts disappear and so SCEV can do the simplification.
Depending on what you want to do, you may want to either do not extend src and dst to 64 bits or add a NSW flag to the computations. That will hopefully make SCEV push the sext outside and then perform the simplification. You can also take a look at ScalarEvolution::SimplifyICmpOperands().

I don’t mind the expression not being simplified (incorrectly!) to -1, but I was hoping that the package would be clever enough to recognize that the expression wasn’t zero.