[Analysis] Simplify the interface to llvm::getICmpCode. NFC

Instead of passing an InstCmpInt * and a bool just pass the predicate
from the caller.

I'm considering moving the similar FCmp functions from InstCombine
over here and this makes the interface consistent with what is used
for FCmp.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D120609