[OpAsmParser] Simplify logic for requiredOperandCount in parseOperandList.

I would ideally like to eliminate 'requiredOperandCount' as a bit of
verification that should be in the client side, but it is much more
widely used than I expected.  Just tidy some pieces up around it given
we can't drop it immediately.

NFC.

Differential Revision: https://reviews.llvm.org/D124629