[InstCombine] less 'CI' confusion; NFC

Change the name of the ICmpInst to 'ICmp' and the Constant (was a ConstantInt) to 'C',
so that it's hopefully clearer that 'CI' refers to CastInst in this context.

While we're scrubbing, fix the documentation comment and use 'auto' with 'dyn_cast'.

llvm-svn: 271817