Remove the now-unused GRState::isEqual method. Instead of asking if an expression...

Remove the now-unused GRState::isEqual method. Instead of asking if an expression equals a certain value, use SValuator::EvalEQ and GRState::Assume to see if it can, must, or must not equal that value.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@107638 91177308-0d34-0410-b5e6-96231b3b80d8