Refactor operation equivalence checking in BBVectorize by extending Instruction::isSameOperationAs.

Maintaining this kind of checking in different places is dangerous, extending
Instruction::isSameOperationAs consolidates this logic into one place. Here
I've added an optional flags parameter and two flags that are important for
vectorization: CompareIgnoringAlignment and CompareUsingScalarTypes.

llvm-svn: 159329