[RISCV] Resolve a few bugs in RISCVVIntrinsicUtils.cpp

This patch does a few things:
  1. Add a new type called Undefined to ScalarTypeKind.
  2. Make RVVType::applyModifier early return when encounter invalid
     ScalarType, otherwise it could be modified to "non-invalid" type in the following code.
  3. When FixedLMULType::SmallerThan is applied, the lmul should be "<" than
     specified one, so lmuls which are ">=" should be marked as invalid.

Differential Revision: https://reviews.llvm.org/D156223