[StructurizeCFG] Clean up some boolean not instructions

In some cases StructurizeCFG inserts i1 xor instructions to invert
predicates. Add a quick loop to clean these up afterwards if we can get
away with modifying an existing compare instruction instead.
(StructurizeCFG is generally run late in the pipeline so instcombine
does not clean them up for us.)

Differential Revision: https://reviews.llvm.org/D118623