[SimplifyCFG] don't create a no-op subtract

I noticed this inefficiency while investigating PR34603:
https://bugs.llvm.org/show_bug.cgi?id=34603

This fix will likely push another bug (we don't maintain state of 'LateSimplifyCFG') 
into hiding, but I'll try to clean that up with a follow-up patch anyway.

llvm-svn: 313829