[SVE] Fix wrong usage of getNumElements() in matchIntrinsicType

I have changed the ScalableVecArgument case in matchIntrinsicType
to create a new FixedVectorType. This means that the next case we
hit (Vector) will not assert when calling getNumElements(), since
we know that it's always a FixedVectorType. This is a temporary
measure for now, and it will be fixed properly in another patch
that refactors this code.

The changes are covered by this existing test:

CodeGen/AArch64/sve-intrinsics-fp-converts.ll

In addition, I have added a new test to ensure that we correctly
reject SVE intrinsics when called with fixed length vector types.

Differential Revision: https://reviews.llvm.org/D79416