[LegalizeTypes] Remove IsVP argument from type legalization methods. NFC

We can either check the opcode or number of operands or use
ISD::isVPOpcode inside the methods.

In some places I've used number of operands figuring that it is
cheaper than isVPOpcode. I've included isVPOpcode in an assert to
verify.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D116578