[RISCV] Add tests showing incorrect BUILD_VECTOR lowering

These tests both use vector constants misidentified as VID sequences.
Because the initial run of elements has a zero step, the elements are
skipped until such a step can be identified. The bug is that the skipped
elements are never validated, even though the computed step is
incompatible across the entire sequence.

A fix will follow in a subseqeuent patch.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D123785