Revert r314017 '[InstCombine] Simplify check for RHS being a splat constant in...

Revert r314017 '[InstCombine] Simplify check for RHS being a splat constant in foldICmpUsingKnownBits by just checking Op1Min==Op1Max rather than going through m_APInt.'

This reverts r314017 and similar code added in later commits. It seems to not work for pointer compares and is causing a bot failure for the last several days.

llvm-svn: 314360