[SVE] Do not store a bool for Scalable in VectorType

Summary:
- Whether or not a vector is scalable is a function of its type. Since
all instances of ScalableVectorType will have true for this value and
all instances of FixedVectorType will have false for this value, there
is no need to store it as a class member.

Reviewers: efriedma, fpetrogalli, kmclaughlin

Reviewed By: fpetrogalli

Subscribers: tschuett, hiraditya, rkruppe, psnobl, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78601