[IR] Value: Fix OpCode checks

Value::SubclassID cannot be directly compared to Instruction enums, such as
Instruction::{Call,Invoke,CallBr}. We have to first subtract InstructionVal
from the SubclassID to get the OpCode, similar to Instruction::getOpCode().

Reviewed By: nickdesaulniers

Differential Revision: https://reviews.llvm.org/D104043