[InstCombine] Allow InstrCombine to remove one of adjacent guards if they are equivalent

This is a partial fix for Bug 31520 - [guards] canonicalize guards in instcombine

Reviewed By: majnemer, apilipenko

Differential Revision: https://reviews.llvm.org/D29071

Patch by Maxim Kazantsev.

llvm-svn: 293056