[LegalizeVectorTypes] Improve SplitVecRes_INSERT_SUBVECTOR to handle subvector...

[LegalizeVectorTypes] Improve SplitVecRes_INSERT_SUBVECTOR to handle subvector being in the high half of the split or not at element 0 of the low half.

This function isn't exercised in lit tests today today according to
the code coverage report. But will be after the tests in D97543 and
D97559.

Posting this patch to help a crash that Fraser hit.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D97582