[CodeGen][SVE] CopyToReg: Split scalable EVTs that are not powers of 2

Scalable vectors cannot use 'BUILD_VECTOR', so it is necessary to
properly split and widen scalable vectors when passing them
to CopyToReg/CopyFromReg.

This functionality is added to TargetLoweringBase::getVectorTypeBreakdown().

This patch only adds support for 'splitting' scalable vectors that
are a multiple of some legal type, e.g.

      <vscale x 6 x i64> -> 3 x <vscale x 2 x i64>

Reviewers: efriedma, c-rhodes

Reviewed By: efriedma

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80139