[CodeGen][SVE] Calculate correct type legalization for scalable vectors.

This patch updates TargetLoweringBase::computeRegisterProperties and
TargetLoweringBase::getTypeConversion to support scalable vectors,
and make the right calls on how to legalise them. These changes are required
to legalise both MVTs and EVTs.

Reviewers: efriedma, david-arm, ctetreau

Reviewed By: efriedma

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80640