[DAG] Fold insert_subvector undef, (extract_subvector X, 0), 0 with non-matching types

We have an existing DAG combine for when an insert/extract subvector pair is entirely a nop, but we hadn't handled the case where the net result was either an insert or an extract (but not both).  The transform is restricted to index = 0 to avoid having to adjust indices after the transform.

Reviews, a couple comments on the test changes:
* Mostly RISCV, mostly schedule reordering.
* One real regression in splats-with-mixed-vl.ll due to a different overly aggressive combine, fix in a follow up patch.
* The test/CodeGen/X86/vector-replicaton-i1-mask.ll diff looked concerning at first, but not the mask size at most 4 i1s.  I think the type changes on the mask loads are correct, but would welcome a second opinion with someone more familiar with AVX512 codegen.

Differential Revision: https://reviews.llvm.org/D158201

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