[SVE] Fix VLS FMA matching for CodeGenOpt::Aggressive.

For NEON, FMA matching is done in the MachineCombiner, and not the
DAGCombiner. That causes problems with VLS lowering, since the
vectors are fixed width at the DAGCombiner, but are scalable in
the MachineCombiner. This patch corrects it by matching FMAs for
VLS vectors in the DAGCombiner.

Reviewed By: paulwalker-arm

Differential Revision: https://reviews.llvm.org/D112557