[CodeGen][SVE] Don't drop scalable flag in DAGCombiner::visitEXTRACT_SUBVECTOR

There was a rogue 'assert' in AArch64ISelLowering for the tuple.get intrinsics,
that shouldn't really have been there (I suspect this was a remnant from when
we expected the wider vector always to have come from a vector CONCAT).

When I tried to create a more minimal reproducer, I found a bug in
DAGCombiner where it drops the scalable flag when trying to fold:

      extract_subv (bitcast X), Index --> bitcast (extract_subv X, Index')

This patch fixes both issues.

Reviewers: david-arm, efriedma, spatel

Reviewed By: efriedma

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82910