Use enum instead of unsigned. NFC.

Use enum instead of unsigned.  NFC.

The unsigned opcode argument here was the result of BinaryOperator->getOpcode().
That returns a BinaryOps enum which is more accurate than passing around an
unsigned.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242265 91177308-0d34-0410-b5e6-96231b3b80d8