[IR] Rename BinaryOperator::init to AssertOK and remove argument. Replace...

[IR] Rename BinaryOperator::init to AssertOK and remove argument. Replace default case in switch with llvm_unreachable since all valid opcodes are covered.

This method doesn't do any initializing. It just contains asserts. So renaming to AssertOK makes it consistent with similar instructions in other Instruction classes.

llvm-svn: 306277