[CodeGen][LLT] Add isFixedVector and isScalableVector (#71713)

The current isScalable function requires a user to call isVector before
hand in order to avoid an assertion failure in the case that the LLT is
not a vector.

This patch addds helper functions that allow a user to query whether the
LLT is fixed or scalable, not wanting an assertion failure in the case
that the LLT was never a vector in the first place.