[AArch64][SVE] Fold target specific ext/trunc nodes into loads/stores

Due to the way fixed length SVE lowering works, we sometimes introduce
ext/trunc nodes very late, these nodes then immediately get converted
into target specific nodes (UUNPKLO/UZP1) before they get a chance to be
folded into a load/store.

This patch introduces target specific dag combines for these nodes so that
we can still create extending loads/truncating stores out of them.

Differential Revision: https://reviews.llvm.org/D128065