[SVE] Remove VectorType::isScalable()

Summary:
* This is a property of the instance of VectorType. If
isa<ScalableVectorType>(T) is true, then T->isScalable() would have
returned true and vice-versa. Most code that checks this function uses
the result to bail out if a vector is a scalable vector. This code will
be cleaner if it just calls isa<ScalableVectorType>(T)

Reviewers: efriedma, craig.topper, huntergr, sdesmalen

Reviewed By: sdesmalen

Subscribers: tschuett, rkruppe, psnobl, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77690