[SVE] Remove IITDescriptor::ScalableVecArgument

I have refactored the code so that we no longer need the
ScalableVecArgument descriptor - the scalable property of vectors is
now encoded using the ElementCount class in IITDescriptor. This means
that when matching intrinsics we know precisely how to match the
arguments and return values.

Differential Revision: https://reviews.llvm.org/D80107