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